Chapter 2: Control System Design Process

2.1. Introduction. 2

2.2. The Dilemma. 2

2.3. The Key Players. 2

2.3.1. The Designer’s Requirements and Challenges. 3

2.3.2. The Contractor’s Requirements and Challenges. 4

2.3.3. The Owner’s Requirements and Challenges. 5

2.4. Tying it Together6

2.4.1. System Diagram and Sequence of Operation. 7

2.4.2. Point List8

2.4.2.1. Benefits of a Points List9

2.4.2.2. Typical Components. 10

2.4.3. Specifications. 12

2.4.3.1. Scope of Work. 13

2.4.3.2. Component Specifications. 14

2.4.3.3. Assembly Specifications. 27

2.4.3.4. Operator Work Stations. 28

2.4.3.5. Installation Requirements. 33

2.4.4. Floor Plans. 36

2.4.5. Standard Details. 37

2.5. Putting These Concepts to Work. 41

 

Figures

Figure 2.1 Detailed Points List 11

Figure 2.2 Typical Detailed Valve Schedule. 16

Figure 2.3 Typical Performance Criteria Based Valve Schedule. 17

Figure 2.4 Typical Detailed Damper Schedule. 19

Figure 2.5 Control System Network with No High-level Controllers. 23

Figure 2.6 Control System Network with High-level Controllers. 24

Figure 2.7 Typical Installation Detail for a Specialized Application. 34

Figure 2.8 Typical Installation Detail for Multiple Trades. 35

Figure 2.9 Mechanical Room Drawing with Control Information. 36

Figure 2.10 Typical Control Panel Detail 38

Figure 2.11 Typical Analog Input Wiring Detail 39

Figure 2.12 Typical specialized terminal strip installation. 41

 

Tables

Table 2.1 Common Air Handling System Points Specified in Contract Documents. 11

 

2.1. Introduction

This chapter provides an overview of a control system design process that has been used successfully to provide well-functioning control systems for numerous commercial, institutional, and industrial HVAC projects. While the process outlined this chapter is not the only viable method for successful control system design, it does provide a useful template that illustrates important design elements. These elements need to be addressed directly and proactively to ensure that the design intent is met. Otherwise, they will be addressed by circumstance or default with results that are far from the desired outcome. The Design Guide uses the template to focus on why and how to specify the elements it contains, thereby guiding designers who wish to modify their existing processes to reflect some or all of the template’s components.

Projects with inadequate control system designs often require additional effort and capital outside of the project budget to meet design intent. Some people may argue that providing detailed specifications will add cost to the project in an industry where budgets are already tight. In this chapter, we advocate that:

·       The capital expended up front during design to ensure the details are adequately addressed will typically be orders of magnitude less than the capital expended subsequent to construction to correct the defects and pay for the inefficiencies that result from lack of design detail.

·       The costs can be controlled by standardizing the proposed methods and by directing the task to those in the best position to accomplish it.

For example, to obtain a functional economizer, someone has to size the dampers, make configuration decisions, and select actuators. Even if the damper is not properly sized, someone will have to make those decisions when it is ordered or installed. While it may take someone with experience to properly size the damper compared to simply ordering it, the time required for the engineering is minimal compared to the cost of modifying or replacing the dampers after they are installed and the operating cost penalties associated with poor performance. On some projects, the designer may be the best candidate for this role. On others, it may be the contractor. What matters is that someone is charged with the task and then held accountable for the results. With these issues in mind, we will start the chapter by examining the working environments for the parties with vested interest in a good control system design. By understanding these working environments, potential problems can be acknowledged and dealt with through a good control system design process.

2.2. The Dilemma

HVAC control systems are the central nervous system of the machine world. The most efficient, well-selected and installed mechanical equipment can be reduced to a problem-riddled, inefficient mechanical nightmare by a poor control system design. One would expect that developing the control system design and installation parameters would be a significant portion of the HVAC design process and that the design of the control system would start early in schematic design. Unfortunately, this process often does not occur, and the control system information is often added to the construction documents at the last minute. In addition, the control system design information is frequently structured in a manner that delegates much of the responsibility for the final product to the control contractor with very little project specific detail included in the contract documents. As a result, the control contractor must develop a price for a final product that is loosely defined in a competitive bidding environment. The result is often less than perfect, frustrating the designer, the contractor, and Owner with problems that can plague the project for its lifetime.

2.3. The Key Players

The key players in the dilemma described above are the HVAC designer, the control system contractor and the building owner. All three of the players have distinct but interrelated needs with regard to the design of the control system. The following sections bring the needs of each party to focus and identify the challenges that each party faces. With an understanding of these issues, the designer is in a good position to lead the efforts to ensure that a robust control system is installed that meets the needs of the owner.

2.3.1. The Designer’s Requirements and Challenges

The rapidly evolving technology associated with current HVAC system design confronts designers with a multiplicity of control system requirements and challenges.

·       Time and Fee Restrictions The technology employed in current building systems has rapidly advanced over the past several decades. At the same time, there are shorter design windows for development of these more complex designs. The fast-paced development and construction schedules and the interdisciplinary interactions required to implement sophisticated building technologies have resulted in the need for frequent meetings between the design team leaders, which has resulted in increased overhead while fees have not increased in a corresponding manner. The net effect is a decrease in the amount of time the design team leaders can devote to project technical development work. To solve this problem, they often delegate tasks to lower level staff that have less experience from which to draw to make crucial technical decisions. Delegating tasks to the contractors via performance specifications has become a common approach for control system design. Unfortunately, competitive bidding pressures often prevent control contractors from implementing the necessary level of quality. The contract language is so vague that it can be interpreted liberally at the expense of quality and performance. With dedication to writing control system technical specifications, designers can successfully create a level playing field for bidders to provide quality controls installations. Specifications are discussed in detail in Section 2.4.3.

In the late 90’s a project engineer working on a hospital surgery addition project was reviewing files from the original, 1940’s vintage, surgery construction project. While reviewing these documents, he discovered that the time allowed for the original project was the same as the time allowed for the current project. Yet, the current project had far more complex issues to consider: The new surgery suite was constructed over an existing outpatient clinic that had to remain in service during construction. The newer project had more complex licensing standards and more complex HVAC, electrical, and utility systems. To top it off, all systems had to be interfaced to the existing plant while it remained in service.

·       Technical Expertise The rapidly evolving field of direct digital control has left many HVAC designers feeling that they lack the knowledge required to develop a detailed control system design. Tight construction schedules and fee structures leave little time or money to support the training required to stay current with the details of control system hardware, programming, and architectural issues for a wide array of manufacturers and suppliers. As a result, many designers delegate many of the details of the control design to the contractors bidding the project, believing that they do not have the time, budget, or expertise to do otherwise. While this may be true with respect to the specific technical details of any given manufacturer’s system, many important parameters related to the design of the control system are actually a function of HVAC requirements the designer is (or should be) familiar with. Examples of these parameters include set points and sequencing, control point locations, and sensor accuracy.

·       Minimize Field Installation and Start-up Problems The HVAC designer has a vested interest in providing plans and specifications that minimize the field installation and start-up problems associated with their projects. Not only does this ensure that the intent of their design is fully realized, it also protects them from liability problems associated with systems that fail to perform to the Owner’s expectations, and improves their ongoing client relationships. For HVAC systems, a good control design is key to successfully achieving the design intent of the project and efficient, reliable system operation. Designers who can develop methods that allow them to clearly and completely describe the control system requirements will be rewarded with systems that start up smoothly and operate reliably. The resulting bottom line will be happier clients, fewer field problems that rob valuable billable production time from other projects, improved client relationships that can foster repeat business and better fees, and reduced liability and exposure to litigation.

2.3.2. The Contractor’s Requirements and Challenges

Control contractors operating in the current construction market find themselves faced with requirements and challenges that are directly related to those that the designer faces.

·       Time and Fee Restrictions The control contractor must develop a low bid for a project based on the information available on the construction documents, usually within a short bid window. The portion of the project that the control contractor bids is technically complex with contractual and physical interfaces to virtually every other contractor working on the project’s mechanical and electrical systems as well as some of the architectural trades. Much of the work associated with a DDC system involves the electrical trades, and many control companies require that their sales engineers perform a detailed electrical take-off for the project or obtain competitive pricing from electricians. The sales engineer or their subcontractors have to develop an understanding of the system architecture and the location of the points relative to the physical arrangement of the equipment on the project. The technical issues associated with the system’s architecture and installation requirements will generally dictate the physical requirements for installing the system. If this information is not well-developed on the contract documents, then the sales engineer will need to develop it within the constraints of the available requirements.

In most cases, while the contractor is developing their bid, the project is still in a state of flux. Last minute changes, answers to bidders questions, and additional clarifications are being added to the contract documents through addendum. Most addendums can add significant contractual and cost for the contractors, yet their scope is difficult to define since they are usually verbal in nature but describe project elements that would normally be described by drawings.

The control contractor’s fee restrictions result from the fact that they are bidding for work in most cases (vs. negotiating without competition). They will not get all of the projects for which they submit a bid, so some costs associated with bidding work must be supported as overhead. Thus, there can be considerable pressure to minimize the amount of effort placed into bidding a project. This bidding environment tends to work against the sales engineer developing well-planned technical solutions to design issues that are not addressed by the contract documents, especially when faced with time pressures.

Vague control system requirements in HVAC contract documents often make it difficult for those bidding to do their best job if they want to get the work. For instance, contract documents that say little about sensor requirements may leave the bidders with little choice other than to select the lowest priced sensor that meets the letter of the documents. Though the paper requirement is met, the sensor may not meet the real operating requirements. Similarly, if system architecture issues are not addressed in the contract documents, competitive bidding pressures will force the bidders to structure the system in the least costly configuration possible. In some cases, this configuration may limit system response time, programmability, and future expansion and flexibility, all of which could become significant issues.

·       Technical Expertise Control contractors commonly find themselves in the position of having technical expertise that they are somewhat powerless to apply. The controls contractors are most familiar with the technical details of the proper application of their control system. In addition, many manufacturers have a staff with considerable HVAC applications engineering expertise. However, if the contract documents do not provide sufficient information to allow the controls contractor to exercise their expertise, competitive pressures to be the low bidder will usually cause the control system to be optimized for price rather than performance, quality, or ease of use. Contract documents that go beyond generic requirements to provide fundamental information and address key HVAC performance issues will allow the control contractors to apply their expertise to provide a bid for a system that truly meets the designer’s intent while still being competitively priced.

·       Minimize Field Installation and Start-up Problems Control contractors are in one of the more difficult positions during the construction and start-up phases of a project. Their submittals are customized to match the specific system configurations associated with the project and cannot be completed until the submittals for other equipment on the project are finalized. The control system installation work is also highly dependent up on the work of the other contractors since the sensors and actuators cannot be mounted until the structures on which to mount them have been constructed. Program debugging cannot begin until the systems are substantially complete and operational. In many projects, weather delays, shipping delays, and other problems cause the installation work to slip behind schedule. If the project completion date is not allowed to slip, the control contractors can find themselves caught between an inflexible project completion deadline and HVAC systems that have not been completed to the point to allow control system installation and debugging to proceed. The control contractor, like the designer, has a vested interest in minimizing installation and start-up delays in order to have adequate time to complete their installation and testing. Their benefits are similar to those realized by the Designer, including fewer warranty problems that disrupt production on other projects, improved customer relationships that can foster repeat business and better profit margins, and reduced exposure to liability.

2.3.3. The Owner’s Requirements and Challenges

Many of the challenges faced by the design and construction team are reflections of the challenges faced by the Owner in developing the project.

·       Time and Fee Restrictions The pressure on the Owner to complete the project and meet the timeline, budget, and market cycles can be intense because the overall success or failure of the project can be tied to these factors. While an Owner may be convinced to spend above budget in one area in order to save money in other areas, this Owner may be unwilling or even powerless to spend money outside of the original project budget. Similar constraints apply to project timelines. Technical problems with dire consequences to the design and construction team, causing them to want to extend the construction timeline or seek additional funds may pale in comparison to the consequences the Owner faces if the project does not come in on time and on budget.

The Owner wishes to foster competitive bidding with a level playing field so that the HVAC system purchased represents the best value for the money while still meeting their operational needs. Systems that are bid as equivalent as a result of vague contract documents, but are not really equivalent on an operational basis can ultimately cost many times the first-cost savings in wasted energy and other operational issues. On the other hand, detailed boilerplate specifications that are not customized to the specific project may provide features that the Owner does not need, increasing first cost without providing a long-term advantage. Inappropriate application of a boilerplate can result in operational problems and inefficiencies due to improper application of systems or components

·       Technical Expertise Even though a sophisticated Owner might have considerable in-house expertise in control systems, the people in those positions seldom have time for more than a cursory oversight role for new construction due to the demands associated with operating the Owner’s existing facility or facilities. A less sophisticated Owner or an Owner with only a few properties may not have in-house control expertise to dedicate to a new project until the project’s operating staff comes on board. As a result, most Owners look to their design and construction team to attend to the details of developing and bringing online the control systems for their new facilities. Ideally, much of the facility-specific knowledge will be transferred to the Owner’s staff during the training that occurs as a part of the start-up process. Contract documents that provide a clear basis for the control system design, supplemented by a good set of control drawings are an important part of the knowledge base that will be provided to the Owner for the life of the building. These documents will be the foundation upon which much of the operating staff’s technical expertise for the building is developed. Thus, the benefits of a well-developed control design accrue to the Owner directly via the construction documents and indirectly via the control submittals that evolve out of these documents.

·       Minimize Field Installation and Start-up Problems More than anyone, the Owner has a vested interest in having a project with a minimal number of installation and start-up problems. Problems of this type can often lead to unanticipated and unavoidable delays in completion of the facility. These delays can ripple into problems gaining the necessary occupancy permits to be able to move in tenants, or delays in production or other beneficial use of the facility, all of which can have an adverse impact on the Owner’s financial plan. It is not unusual for start-up and installation problems to be only partially resolved. These unresolved issues then become ongoing operational problems and inefficiencies that require expenditures from the facility’s operating budget to correct or they simply plague the operators and the tenants for the life of the facility. The benefits that accrue to the Owner from a control system design process that minimizes installation and start-up problems include fewer warranty and ongoing operational problems and improved customer relationships with tenants, which can translate into long-term leases and better rates. For production facilities, minimizing installation and start-up problems causes fewer disruptions to the production process, which results in better profitability. In both types of facilities, minimizing these problems reduces the Owner’s exposure to litigation, both in defending themselves from disgruntled tenants seeking damages as well as in seeking restitution from errant designers and contractors.

In the late 90’s, semiconductor manufacturers experienced a sudden and unexpected economic downturn. One manufacturer was completing a 250,000 square foot expansion when the downturn occurred. The facilities group was baffled when management passed up some attractive energy savings opportunities in their drive to maintain schedule, because implementing them might have delayed the start-up of production. The facilities group became painfully aware of the reason behind management’s drive when the facility was closed one week prior to initiating production. Even though the plant was on time and on budget, it had missed going into production during the peak of the market cycle, and the manufacturer chose let the plant sit idle.

2.4. Tying it Together

As can be seen from the preceding section, the designer, contractor, and owner have interrelated requirements and challenges associated with a project’s control system. All parties have interwoven time and financial constraints under which they all must operate. The owner initiates the project to meet some specific need, often with specific technical requirements that must be met in order to be successful. Often, these technical requirements have evolved out of operating experience on other projects. The designer must understand these needs and translate them into mechanical systems that, if properly controlled, can meet them. The contractor must interpret and implement the designer’s control requirements per the design intent, thus satisfying the operational needs set out at the start of the project by the Owner. Obviously, all parties benefit from a process that provides a smooth installation and problem free start-up.

Designers can take a significant step towards ensuring that their control system needs are met by incorporating certain key elements into the control system design either by including them directly on their contract documents or by requiring that they be furnished as a part of the control system submittal package (and then verifying that they are in fact provided and well executed). These components include:

1   System Diagram and Sequence of Operation A System Diagram is a schematic drawing of the arrangement of the entire system to be controlled including all interacting components. The location of all control system inputs and outputs associated with the HVAC system should be included on this diagram. In a narrative format, the sequence of operation describes the required HVAC control process in detail including all operational and interlock requirements.

2   Benefits of a Points List Each physical point on the project as well as key virtual points should be identified on a points list. The list should include important parameters such as sensor accuracy requirements, alarm limits, and trending requirements.

3   Specifications The performance and installation requirements for the sensors, actuators, final control elements, controllers, workstations and other components that comprise the control system are detailed in the specifications. Control damper and control valve schedules are an important component of these specifications.

4   Floor Plans The location of the input and output points as well as key components like control panels, operator workstations, major cable and conduit routes, and sources of power are shown on control system floor plans.

5   Standard Details Typical installation requirements for the control system elements are provided in standard details. These details can illustrate the general design intent for the system, provide a consistent basis for estimating the control price, provide the basis for the system-specific control system design development by the control contractor, and provide guidance for the other trades involved on the project who must coordinate with the control contractor.

The specification, system diagram, and operating sequence will exist in some form on just about all projects. Other elements, such as the point list, floor plans, and installation details may not be part of the normal scope of design services and may not even show up on the contractor’s submittals. However, once some experience is gained, these components are surprisingly easy to generate, especially in an office that utilizes a CAD package and word processing to automate tasks. As a result, some designers are electing to include them with their design package while others are including language to delegate them to the control contractor, thus ensuring that they are attended to. These elements generate rewards through improved communication of the design intent, better bidding, fewer start-up and operational problems, which will more than pay for any added costs that may be incurred. Return on investment is also realized through improved client relationships, improved fee structures, and reductions in the often non-billable time associated with resolving disputes and handling field problems during the construction process and warranty year. The following sections will discuss each of the six components of the control system design process listed above in greater detail.

2.4.1. System Diagram and Sequence of Operation

Successful HVAC designs hinge on the smooth, integrated interaction between the system’s components and the loads served. It is not just an air handling unit; it is an air handling system made up of an air handling unit, intake system, distribution system, terminal equipment, return system, relief, and exhaust system. It is critical that the control system design reflect this systems-based perspective. It is this systems perspective that guides creation of the system diagram and the detailed sequence of operation.

The system diagram is a drawing that shows the entire system under consideration in schematic format, not just portions of the system. This method allows the user to see the entire process and visualize the potential interactions without having to flip between multiple documents. A detailed system sequence of operation or system narrative goes hand in hand with the system diagram in documenting the overall operation of the system. Many times, the sequence provided on the contract drawings and duplicated in the specification provides a good overview of how the system is intended to perform, but fails to address critical details which can make or break the success of the installed system. Well-documented system drawings also provide a useful field reference for the commissioning and operations personnel. More information and examples of the systems perspective is provided in the Functional Testing Guide for Air Handling Systems (Functional Testing Guide) in Chapter 2: Functional Testing Basics.

Most designers will find that the schematics and operating sequences typically included on their construction documents can be readily adapted to reflect the system concept, often with little effort and a great deal of benefit for an improved design process and improved installations. Often, the difficult part of this transition to the system concept is learning to write out the detailed operating narrative. This detailed narrative is just a written statement of what the designer should already know: the details of how they expect the system to function. Furthermore, the initial development effort can often be amortized over subsequent projects since many of the operating requirements for HVAC systems do not vary from project to project. This allows a designer to develop standard sequences for typical system configurations that can then be adjusted as necessary to the specific requirements of a project.

In addition to the ongoing usefulness of a well-written sequence of operation to the building operators, the sequence is essential for a smooth commissioning process. The commissioning provider must test and evaluate the system sequence based on how well it meets this detailed sequence.

Some consulting firms have found that the system diagram and narrative sequence of operations are such an integral part of a successful design that they are required as the starting point for any new project. Before a project engineer can request drafters and designers for a project, they must have developed a system schematic, a fairly detailed narrative describing how the system should work, and a rough estimate of the heating and cooling loads. If this process is well-executed, a significant portion of the engineering required to make the project successful is complete or will fall out of the information developed. The technique can yield preliminary equipment selections and motor loads within 10-15% of final values, as well as parameters for estimating costs and equipment room, mechanical chase, and ceiling space requirements. This type of information can also be used to coordinate and negotiate with the other project team members based on a foundation of technical information, not rules of thumb.

Similar considerations apply to the system diagram. For most design firms, a schematic rendering of their standard systems will not vary much from project to project because the schematic arrangement is fairly independent of building geometry. This means that a design firm can develop standard schematics for the systems configurations that they use repeatedly and then tailor these standards to the needs of specific projects.

The current technology automated office environment makes all of this easy to accomplish using standard word processing and computer automated design and drafting techniques. Developing a project’s system configurations and operating sequences from existing standards does not remove the need for technical assessment and expertise to adapt the standards to the specific needs of a project. It does minimize the amount of time that must be spent in this process by avoiding “reinventing the wheel”.

2.4.2. Point List

Both the narrative sequence and the system diagram provide a basis for developing the points list. Some designers incorporate the point list into the narrative sequence, while others provide it as a table in the specifications or contract drawings or by showing the points on the system schematics. Regardless of where the point list is located, it is a good idea to show the points in the proper location on the system diagram to guide the contractors during construction.

The point list should include all physical points on the project, including hardwired interlocks. It is also a good idea to include required virtual points such as calculated flows or energy consumption. Other virtual points, such as set points and tuning parameters can also be included, but these points can usually be accommodated with less effort by the designers through a specification reference or a general note attached to the point list.

While some commissioning and O&M-related points may not directly affect the ability of the control system to function, they provide a means to ensure that the system continues to perform as intended by allowing key performance parameters to be monitored, trended and alarmed. When coupled with an ongoing commissioning plan and good training for the operating staff, monitoring points enable the design intent and design efficiency of the HVAC systems to persist over the life of the facility.

2.4.2.1. Benefits of a Points List

A point list can provide a great deal of information regarding the requirements for the control system. Detailed specifications of control and monitoring points can be beneficial to both designers and controls contractors in the following ways:

·       Without a points list, the determination of points is left to the contractor’s interpretation of the contract documents. Often points are left out that are not necessary to execute the sequence but are useful for future sequence modifications, building control loop tuning, energy consumption analysis, and O&M troubleshooting. Adding the points later can be difficult and expensive.

·       By clearly stating the minimum point requirements for the project, the designer can ensure a “level playing field” for all control system bidders.

·       Requiring specific points enables the bidders to more easily obtain pricing from electrical contractors and equipment suppliers.

·       The point list can be used to tie together information about the point location and function on the system diagram with information about its physical location in the building. This can be especially helpful if the points are not shown on a floor plan.[1]

·       The point list provides a place to clarify the requirements for sensor accuracy, sensor configuration, and any other special requirements.

·       The points list reveals requirements of the control system architecture. The point density required in a particular location may determine the controller network configuration and the I/O requirements for the controller.

·       The points list can be developed early in design to provide a starting point for the control system budget.

Computer-based control systems employ two classes of points: physical points and virtual points. Physical points exist as hardware devices, typically sensors or actuator. They are wired to the controller I/O to allow the control program to execute the intended functions and provide information for diagnostics and troubleshooting. In contrast, virtual points exist in the controller’s memory and are used to store set points, counter and timer values, perform calculations, and act as logic flags. They may also represent physical quantities such as a flow rate calculated based on a differential pressure signal or a Btu consumption calculated from a flow rate and a temperature difference. Not all virtual points are important for automatic control. However, the information provided by virtual points can be very useful to operators. It is important is that the designer ensures that the details associated with the virtual points are a part of the turn-over package provided to the Owner once the system is commissioned and functioning properly.

2.4.2.2. Typical Components

In addition to listing the minimum points requirement for the project, the point list is a convenient way to specify the requirements associated with each point, such as sensor type, accuracy, and point name. An example point list is shown in Figure 2.1. The spreadsheet used to create this figure is also provided as a starting point for developing your own point list. Providing a useful points list is critical, but there are many ways to go about doing it. Some may feel that the attached example contains too much detail, the detail should be located elsewhere, or that the contractor should provide some of the information.

The following items are suggestions for consideration when developing your point list format.

·       Point Name and Symbol The point name provides a consistent way to reference the point in the contract documents and correspondence. It may also be convenient to add a suffix indicating if the point is part of the base bid or an alternate where appropriate.[2] (Other point naming convention considerations are discussed in Section 3.6.1.) Some designers find it convenient to combine the point name with a drawing symbol for different point types and use this information to designate the point location on the system schematics and floor plans.[3] (See Figure 2.9 for an example of this.) Project personnel will quickly learn to interpret these codes and will appreciate the utility they bring when reading the control drawings.

·       Applicable Details Applicable details point the contractor or installer to drawings that further describe the point installation. A column to indicate drawing details has not been shown on the example points lists due to space constraints but is included in the template.

·       System and Service The full name of the point and the system that it corresponds to. For many systems, this can become the point descriptor.

·       Sensor Type and Accuracy A minimum level of accuracy should be specified. Requirements for sensor type and accuracy are presented by application in Section 3.3 Sensor Accuracy.

·       Limit and Warning Alarm Requirements See Section 3.6.4 Programmable Alarms for details about selecting alarms.

·       Trending Requirements See Section 3.6.6 Point Trending for more information on trending during the commissioning process and during normal operations.

·       Bid Level Section 3.2 Point Selection can assist designers in selecting points to include as a base bid or alternative bid level. A column to indicate bid level has not been shown on the example points lists due to space constraints but is included in the template.

·       Notes The notes column indicates details that will help the project personnel understand the purpose of the design requirements.

Figure 2.1 is an example of a typical point list where the designer specified the project point requirements in detail. Alternatively, the designer may specify only the points required and refer the contractor to the specification for some of the requirements. In this case, the designer may require that the control contractor provide a detailed point list as a part of the submital process.

Bevel: Link to Detailed Points List Spreadsheet

Link to a detailed points list (Excel spreadsheet). Save this spreadsheet to use as a starting point for point lists on your projects.

Figure  2.1 Detailed Points List

Table 2.1 lists point types that are commonly found on air handling systems. Each of these categories of points should be included in the points list.

Table 2.1 Common Air Handling System Points Specified in Contract Documents


Analog Inputs

Digital Inputs

Analog Outputs

Digital Outputs

Virtual Points

System discharge temperature

Fan proof of operation

Economizer damper command

Fan Start/Stop

Set points

Mixed air temperature

Filter status

Relief damper command

Drive Enable/Disable1

Calculated flow rates

Coil leaving temperature

Humidifier proof of operation

Face and bypass damper command

Humidifier shut down

kWh consumption

Zone temperature

Preheat coil pump proof of operation

Hot water valve command

Preheat coil pump command

PID constants

Zone humidity

Selector switch status

Chilled water valve command

 

 

Duct system pressures

Override switch status

Humidifier valve command

 

 

Building and zone pressures

Low temperature limit status2

Drive speed command

 

 

Outdoor air conditions

Mixed air plenum static safety status2

Inlet vane command

 

 

Return air CO2 level

Discharge static safety status2

 

 

 

Valve and damper actuator positions

Return static safety status2

 

 

 

Direct flow rate measurement

Power status3

 

Power status3

 

Terminal zone flow rates

Fire alarm status

 

 

 

Note 1 There is a subtle difference between a start/stop point and an enable/disable point. A start/stop point will directly control a piece of equipment; when the command is sent, the equipment will immediately respond. In contrast, an enable/disable point is a permissive command that allows some other control system to start or stop the equipment based on its internal parameters. A Start/Stop command might be used to cycle a single speed fan or pump controlled by a starter. An Enable/Disable command might be used to cycle a fan or pump that are controlled by a VFD where the start sequence programmed into the drive might include a DC injection braking command prior to the start command.

Note 2 The actual safety control should be hardwired into the common side of the related interlock circuit. To save point capacity, a general safety status point can be provided instead of specific points for each safety device.

Note 3 Power status is a relay contact wired to the same power source as the control and fan system. The point triggers a power recovery restart routine to safely restart the system after a power failure. Many controllers now have this feature built into them so they can accomplish the function without using any of their I/O.

2.4.3. Specifications

The technical specifications associated with the control system are an important part of the overall controls design because they contain many of the technical details that are critical to success. The function and content of the control specification are just as important as other mechanical sections. For instance, most piping specifications contain detailed descriptions of the valves, pumps, chillers, piping specialties, and other components used to build up the piping system. Designers would not put a project out to bid without this important information for the piping system, but this kind of detailed information is not typically included for control system. The lack of a detailed control system technical specification can lead to difficulty meeting the owner’s design intent.

Control technology has changed so quickly in the last 15 to 20 years that it got ahead of the standard technical specifications used by most engineering firms. While there have also been technological changes in mechanical systems, they have generally been more easily accommodated by making modifications to the fairly detailed mechanical specifications that already exist. For instance, the application of variable speed drives to variable flow fan and pumping systems required that either the mechanical or electrical designer develop a specification paragraph for the drive itself and perhaps make adjustments to the motor, fan, and pump specifications that they already had in place. In contrast, the evolution of control systems from networks made up of pneumatic, electric, and electronic components to computer based DDC systems required modifications to nearly every paragraph of a typical control specification section and additional requirements. Not only did the technology change, the materials and methods required to implement it and the language required to describe it changed. As a result, the responsibility for implementing the details of the controls design is often delegated to the controls contractors, either directly by performance specifications or indirectly by a simple lack of detail in the contract documents. The contract language for control systems is often so vague that it can be interpreted liberally at the expense of quality and performance. With dedication to writing detailed control system technical specifications and then enforcing them, designers can successfully create a level playing field for quality controls installations.

Designers that are uncomfortable with writing specifications for the latest technology in DDC control systems should consider the following points:

·       Most of the HVAC processes that are controlled by current technology DDC systems were successfully implemented with the control technology that existed prior to the advent of DDC. Successfully defining the control system design revolves around HVAC and mechanical parameters, regardless of the control technology. Thus, if designers can learn to communicate these HVAC needs clearly, significant improvements in performance will be realized regardless of whether or not the designer is fully fluent in networking, programming, and other DDC control implementation strategies.

For example, the system designer may not know whether Ethernet, ARC Net, or a proprietary networking strategy is the best way to achieve their design intent. But they should know that for their system to perform as required, the control system would need to monitor certain points within a certain accuracy tolerance and distribute the information within a certain time frame. By conveying these important design details to the contractor through the contract documents, the designer helps ensure a good control installation regardless of their level of familiarity with the details of DDC controls design or the specifics of a particular manufacturer’s system.

·       Most of the new control technologies come back to the same physical principles that apply to the older technologies. Thus, those who are well-versed in HVAC processes can understand any new control technologies to the extent necessary to develop a good control design. All control and sensor technologies come back to fundamental concepts. For example, chilled mirror technology for measuring dew point functions by cooling the mirror to the point where dew is formed and detecting this point. The details surrounding exactly how this process is achieved are not necessary to specify the sensor type for a particular application.

The following sections will give designers guidance in creating a level of detail in the control system specifications that will help ensure that quality control systems are implemented.

·       Scope of Work

·       Component Specifications

·       Operator Work Stations

·       Installation Requirements

2.4.3.1. Scope of Work

In addition to a general statement of work for the control system scope, some designers find it desirable to include more specific references to the applicable components of the overall scope. These references can be a particularly useful guide since the control work often touches the work of many other trades and sections of the specifications. General areas of control system work often include:

·       Components of the control system such as controllers, workstations, sensors, actuators, and auxiliary control panels and equipment.

·       Conduit and wiring required for the control system inputs, outputs, and network communications. Power wiring for the controllers and actuators and terminal units should also be included if it is not covered by other sections.

·       Control air piping including a source of supply if pneumatic actuation is used and an existing control air system is not available.

·       Equipment interfaced to the control system including chillers, boilers and variable speed drives.

Coordination with other specification divisions and trades that must interface with the control system is essential. This coordination must occur among:

·       The commissioning provider, who will be involved in the check-out of the equipment and control system.

·       The mechanical trades that mount the sensing and calibration wells, pressure sensing ports, control valves, flow meters, and other accessory equipment required by the control system.

·       The sheet metal trades who often install dampers.

·       The electrical trades that typically furnish and wire the electrical starters, which are interfaced to the control system. In addition, the control contractor often furnishes safety switches that are installed by the electrical trades.

·       Engineering and technical support as required to develop and supervise the shop drawings, database, and programming.

Because the control work touches the work of many trades, it is desirable to insert language giving specific instruction regarding the interface between that trade and the control contractor. For instance, the piping section in the specification may need to layout specific requirements for the installation of flow meters, control valves and other control equipment that is furnished by the control contractor. For example, some flow meters require special jigs to ensure accurate alignment of the sensing heads when they are welded to the pipe, and these requirements should be included in the specifications.

2.4.3.2. Component Specifications

Many designers would argue that they simply do not have the fee, time, or expertise necessary to develop detailed component specifications for the control system. Much of the control system component quality control will be achieved via the specification language. Failure to expend effort in component specification will often compromise the quality of the control system and impede the ability of the overall design to achieve its design intent.

Sensors and Actuators

A control system is only as good as the sensors that provide information to it and the actuators that allow it to interface with the equipment. Thus, the specifications for these components are critical. The controller Input/Output (I/O) circuit boards provide the interface between the DDC processors and the sensing and actuating system. While it is not necessary to understand or specify the details of how this interface works, it may be desirable to include language in the spec that covers some features in this area. Topics to consider include:

·       I/O Modularity Some controllers use a modular I/O approach to allow them to be configured to the exact requirements of a project by selecting the correct assortment of input and output modules. Other controllers utilize circuit boards with a fixed number of points with specific capabilities.[4] These features may be important to an Owner or designer in the context of future expansion capability, adaptability to HVAC system modifications, or maintenance requirements.

·       Processor Modularity Some controllers have all of their electronics including the processor and I/O hardware mounted on the same circuit board. Others have modules for each of the major functions. The single circuit board approach tends to hold first costs down, but can represent a significant maintenance cost in terms of materials, labor and downtime since the entire controller will be unavailable and must be replaced if any single component fails.

·       Override Capability One important feature that may only be available on certain controllers in a particular manufacturer’s product line is the ability to manually override an output. If the manual override is a standard feature on the output circuit boards, it often includes the ability to monitor the status of the override switch. Alarms can then be generated if a controller is taken out of the auto mode. The operator can also verify that each output is actually in auto. This feature is useful during start-up and commissioning and in emergencies.[5]

Sensor specifications should address every type of input that is required on the project from the analog temperature and pressure sensors to the digital current sensors and filter switches. Some sensor types may involve multiple specification paragraphs to define the accuracy or sensing element requirements for different applications. For instance, some projects may require two different immersion temperature sensor specifications, one for hot water applications where larger operating spans and lower accuracy is acceptable, and a second paragraph for chilled water applications where small spans and high accuracy may be necessary to achieve the desired degree of precision. Similarly, a duct temperature sensor specification may require several sub-paragraphs dedicated to single point elements, rigid averaging elements and flexible averaging elements.

Actuator specifications also need to be tailored to each project. In many instances, the actuator specifications will be included with the specifications for final control elements like dampers or control valves. Regardless of where they occur, the specifications should address power source (pneumatic, ac or dc electric), actuating force, actuating speed, precision, position feedback requirements, shut-off requirements, and auxiliary requirements such as positioning relays and limit switches. Where DDC controllers will drive pneumatic actuators, specifications regarding the electronic to pneumatic interface devices should also be included. Important items to address might include the required output span, requirements for a gauge to indicate output pressure, and the ability to manually override the output.

Additional information regarding sensors and actuators can be found in Chapter 3: Selection and Installation of Control and Monitoring Points.

Final Control Elements

The most common final control elements in current technology HVAC systems are control valves, control dampers, and variable speed drives. Each of these components require special attention in the specification language. The control work for these components must coordinate with the work of other trades and other sections of the specifications since the elements might be:

·       Furnished and controlled under the control specification section but installed under other sections or divisions of the specification.

·       Furnished and installed under other sections or divisions of the specification and controlled under the control section.

·       Furnished, installed and controlled under the control section of the specification.

The multi-disciplinary aspects of these elements may also require special attention to coordination during design since the design work for different sections or divisions of the specification is often done by different design firms that specialize in a particular area.

Control Valves

Regardless of who furnishes the control valves for a project, important parameters must be addressed in specific terms in order for the design to be successful. These parameters include:

·       Materials of construction

·       Temperature and pressure ratings

·       Actuator power source, failure mode upon loss of power, and operating speed

·       Maximum shut-off differential pressure requirements

·       Sizing

Ideally, the designer should address all of these parameters directly in the design documents, but there are other options. Again, what matters is that the details be addressed by someone who is held responsible for the outcome. A valve schedule with a line for each valve on the project and a column for each parameter to be specified is an ideal way to address the details of valve sizing and selection. This could be included on the drawings or as a part of the specifications. Figure 2.2 is an example of a typical valve schedule where the designer specified the valve design requirements in detail. Figure 2.3 is a similar schedule but has been used by the designer to specify the critical performance criteria, leaving final selection and development of the details of the schedule to the control contractor. The link below will take you to copies of these examples as well as a blank version of the spreadsheet which will calculate valve Cvs based on stanadard ASHRAE equations if you provide the flow and pressure drop targets associated with your design.

Bevel: Link to Valve Schedule Spreadsheet

This spreadsheet is a blank version of Figure 2.2 and Figure 2.3. You can use this spreadsheet as a starting point for valve schedules on your projects.

Figure 2.2 Typical Detailed Valve Schedule


In this example, the designer has sized and selected the valves in addition to specifying the key performance parameters integral to achieving the design intent for the project. This approach allows the designer to assert the most control of the valve selection process but also requires a higher level of expertise and a more significant time commitment during design than the approach illustrated in Figure 2.2.

Figure 2.3 Typical Performance Criteria Based Valve Schedule


In this example, the designer delegated the details of the valve sizing to the control contractor based on critical design criteria specified in the valve schedule. The flow coefficients are targets calculated by the spreadsheet based on ASHRAE sizing equations and the criteria specified by the designer. The designer could elect to simply fill the flow coefficient column with Note 6 and let the control contractor make all the valve design decisions. However, the calculations are fairly simple, especially when automated using the spreadsheet provided. Including these criteria gives the designer more leverage to ensure that the correct valve is supplied.

Many product lines have a “hole” in the range of Cv values available. If a supplier charged with sizing the valves discovers that the Cv requested for a particular valve falls in this “hole”, they are often tempted to use the closest larger size rather than go to a competitor’s product line that has an appropriate Cv. The resulting oversized valve can lead to control problems in the long run. Including a target Cv in the designer-specified parameters helps drive home the point that the valves need to meet the performance requirements of the project, not the product capabilities of the supplier’s product line.

If time or budget precludes this depth of involvement on the designer’s part, then the decisions should be clearly delegated to the contractor and held to acceptable performance criteria. A designer might include performance specifications by specifying that:

·       Materials of construction and temperature and pressure ratings be equivalent to what is specified in the piping section of the specification for a valve serving the same system.

·       The actuator power source, operating speed, and failure mode be consistent with the requirements of the control sequence and the overall actuation strategy that the control contractor is using on the project. The strategy (for instance, pneumatic actuation using electric-to-pneumatic signal converters) may be specified by the designer or left to the discretion of the control contractor.

·       The valve actuators have sufficient actuating force to shut off against the peak pressure on the pump curve for the pumps associated with the system plus a safety factor.

·       The valves for modulating service are sized for a pressure drop that is equal to the design flow pressure drop through the load they serve ±10%.

·       Two-position valves are line size and selected for minimal pressure drops when wide open.

Installation requirements can often be addressed by general specification language supplemented by a standard detail. Language and details associated with installation requirements will need to be included or referenced in the specification sections and drawings for the trades installing the valves.

Control Dampers

Considerations similar to those stated in the preceding section with regard to control valves also apply to control dampers. Parameters to include in the specification language are

·       Materials of construction

·       Blade configuration (parallel or opposed, air-foil or flat plate)

·       Leakage requirements

·       Torque requirements (both for actuation and to ensure that the leakage specifications are met)

·       Actuator power source, failure mode upon loss of power, and operating speed

·       Sizing

As was the case for control valves, the designer can address each of these issues specifically for each damper on the project via a damper schedule or some other means. No matter what the process, it is important that someone is charged with taking care of the details associated with the damper selection and sizing.

The link below will take you to a spreadsheet that you can use as a starting point for developing your own damper schedule. It includes an example of the calculations used to size the minimum outdoor air, economizer, and relief dampers as well as damper characteristic curves. However, you can also address these issues in more general terms in a manner similar to that described for control valves in Figure 2.3.

Bevel: Link to Damper Schedule Spreadsheet

This spreadsheet is a blank version of Figure 2.3 and Figure 2.4. Use this spreadsheet as a starting point for valve schedules on your projects.

Figure 2.4 Typical Detailed Damper Schedule


Damper installation requirements may be slightly more complex than those associated with control valves because parameters such as blade orientation relative to the air stream, blade rotation, actuator location, actuator linkage arrangement, and multi-section reinforcement requirements should be specified. These details are especially important in mixing applications or for large, multi-section dampers. If time and budget permit, the designer can detail each mixing damper assembly to the extent necessary. But the general requirements could also be conveyed via standard details with language in the specification delegating the responsibility of detailing each damper assembly to the control contractor. Any details included in the drawings should be supplemented by specification language. As discussed for control valves, any installation detail requirements need to be included directly or by reference in the drawings and specifications governing the work of the trades that install them.

Additional information regarding control dampers can be found in the Functional Testing Guide, Chapter 3: Economizer and Mixed Air, Section 3.4.1.1 Damper Oversizing.  

Variable Speed Drives

·       Variable speed drives are seldom furnished under the control section of the specification. The language required to adequately define the drive is well-developed and thus will not be discussed here. However, regardless of who furnishes the drives, the control contractor will usually need to interface to them to properly execute the control sequences. The coordination requirements associated with this interface need to be addressed clearly by the control system design. Usually, these requirements can be adequately accomplished through specification language, but a supplementary standard detail is a desirable addition to convey intent for bidding purposes. Interface requirements to be addressed include:

·       Safety device interlock requirements that function in all operating modes (hand and auto as well as inverter or bypass for drives equipped with bypass capabilities).

·       DDC start-stop command interlock requirements Should the start-stop command operate in inverter mode and bypass mode, or just inverter mode?

·       Drive to DDC system interface technique Should hardwired data points be provided or should a communications-based interface that allows the drive to communicate with the DDC system at a communications bus level be provided?

·       Drive programming requirements and responsibilities Define who sets and adjusts the programmable settings in the drive such as acceleration and deceleration times and DC injection braking settings as required to tune the drive to the control program and HVAC system.

·       Control loop programming and parameter locations: Will the control loop program run on the drive circuit board or in the DDC controller? Many drives can accept an input signal from a sensor, a set point signal from a control system and then can execute a PID control loop directly in their on-board microprocessors, making the drive control relatively independent of the control system. In a more traditional approach, the controlling input is wired to the DDC controller and runs the PID loop at that location. An output from the controller is then used to modulate the drive.

As with the other final control elements, any installation details need to be included directly or by reference in the drawings and specifications governing the work of the trades that install the drives.

Additional information regarding variable speed drives can be found in the Functional Testing Guide, Chapter 12: Fans and Drives.

The inputs and outputs associated with a control loop should be connected to the same circuit board that is running the loop program. This avoids problems that can occur when the control system communication bus becomes a part of the control loop. Consider the case in which a static pressure sensor wired to the DDC system is used to control a supply fan drive, and the drive uses its internal program for control based on this static pressure data. The DDC system must transmit the pressure data to the drive via the communications network. The rate at which this data transfer occurs can vary as a function of other network activities, introducing a variable time constant into the control loop, which can cause tuning problems. In addition, a communications network failure results in a control loop with no input, a potentially dangerous condition. In contrast, if the static pressure sensor was wired directly to the then the control loop would be immune to the effects of communications failures or network activity levels.

Controllers and Architecture

An entire design guide could be dedicated to the subject of DDC controllers and system architectures. A knowledgeable designer could develop extensive specifications documenting the specific requirements in this area. However, compared to not addressing the control system controller and architecture requirements at all, significant improvements can be made to control system specification language by addressing the key issues described in the following paragraphs.

Application Specific Controllers vs. Fully Programmable Controllers

Not all DDC controllers are created equal. In addition to having varying point capacities, processing speeds, and communications speeds, the programming capabilities of different controllers can vary significantly.

Application specific controllers have many of the standard HVAC operating strategies pre-programmed into their memory. This programming simplifies the installation and start-up process because the installing contractor simply selects which algorithms need to be executed to match the specified sequence, fills in input and output names and set points, and the system is functional. This approach minimizes start-up bugs since many of the problems were worked out when the installed applications were developed and “burned” into the controller’s memory. However, the application-specific controllers can only execute the functions for which they have been preprogrammed. If the controller is applied to a customized system, this limitation can be significant. The control logic required to execute specialized functions becomes complex because it becomes necessary to “fake” the controller into performing the desired sequence. The manipulations to the existing sequences may not be obvious to an operator troubleshooting a problem at a later date. In addition, the lack of flexibility associated with an application-specific controller can limit the adaptability of the system to handle problems during start-up and future changes in a manner that is not addressed by the “canned program”. The first cost advantages and simplicity offered by application specific and lower level controllers should be carefully weighed against this loss of flexibility for handling start-up problems and future system configuration changes when project specifications are developed. Generally, this type of controller is desirable for smaller systems and terminal equipment where the requirements are standardized and unlikely to change.

Larger central systems and equipment are often best served by a fully programmable controller. When using a fully programmable controller, all program logic must be developed for the project and customized for the application.[6] As a result, programming costs will be higher for this type of controller. Start-up and debugging of these controllers will also tend to be more expensive than an application specific controller due to the customized nature of the program. However, these controllers offer nearly unlimited flexibility (given sufficient memory and point capacity) that can be valuable during start-up and over the life of the HVAC system.

Polled Communications vs. Peer-to-Peer Communications

Controllers on a peer-to-peer network communicate interactively with each other at a equal or nearly equal level; i.e. as peers. Peer-to-peer controllers exchange information in a manner that gives each controller the ability to “talk” or “listen” to the information presented on the network by the other controllers. A network that is operates in this manner will generally will be more robust and generally faster than a polled network.

On a polled communications network, inter-controller communications are controlled by a supervisory interface. For one controller to share information with another controller on the network, the supervisory interface must first “ask” the controller with the desired data for the required information, then the supervisory interface must “tell” the controller that needs the data the information. This process will occur even if the two controllers that need to exchange the data are physically wired together. In addition, the lower level controller generally must wait for the supervisory controller to ask it for information even if this information has changed significantly. While both of these processes tend to reduce the cost of the controller, they also slow down the system and make it less responsive. The number of controllers on the lower level network can also have a significant impact on communications speed because the supervisory interface becomes a communications bottleneck when the number of controllers it must supervise is large.

Both peer-to-peer and polled communications may have an appropriate place in any given control system. Polled communications will generally be satisfactory for controllers associated with small machinery and terminal equipment where the need for information from the network is modest. However, a polled communications strategy can become a hindrance if it is employed for larger controllers with high point densities controlling interactive machines, especially if the system needs to handle a significant amount of data for trending or monitoring. In this type of application, peer-to-peer communications will provide superior performance.

High Level vs. Low Level Controllers

The network architectures provided by the various manufacturers generally include two levels of controllers. Not all manufacturers have systems with controllers at both the high and low levels. Figure 2.5 and Figure 2.6 illustrate this for two different systems.

Figure 2.5 Control System Network with No High-level Controllers

Image courtesy of the DDC Online Web Site (www.ddc-online.org)

Figure 2.6 Control System Network with High-level Controllers

Image courtesy of the DDC Online Web Site

The following features distinguish high-level controllers from low-level controllers:

·       Memory All controllers will have programmable and non-programmable memory. The non-programmable memory is used for the controller’s operating system and other built-in logic that allows the controller to function and communicate. In application specific controllers, the non-programmable memory will also include the HVAC application programs for which the controller was designed. The programmable memory includes the user-generated programming as well as other data like the point data base and trends. High-level controllers tend to have significantly more memory than low-level controllers, and often have the capability to expand the base memory allocation by the addition of memory chips or cards. In most control applications, you will never have too much memory, so it is wise to specify that all controllers be provided with the maximum amount of memory available. Memory that is unused for programming purposes is available for trending, a powerful commissioning and operational tool.

·       Communications Most low-level controllers communicate over relatively low speed, polled networks and are subject to the communications limitations associated with that strategy. High-level controllers will communicate over high speed peer-to-peer networks, making them robust and responsive.

·       Programming Capabilities Generally, high-level controllers will be fully programmable devices with a wide range of functions available to them in their program logic set. In contrast, low-level controllers may be fully programmable. If lower level controllers are fully programmable, they may have limited logic capabilities compared to a higher level, fully programmable controller. For instance, basic math functions such as addition, subtraction, multiplication and division may be supported, but more advanced functions like powers and roots may not be supported. This may limit the usefulness of the controller even if it is fully programmable.

All of these topics are discussed in detail in two excellent resources:

1   An article in the May 2001 issue of Heating, Piping and Air Conditioning titled All Controllers Are Not Created Equal - Knowledge Of The Differences Is Key To Specification by J. Jay Santos, PE is available for download from the HPAC website at www.hpac.com. This article discusses the various types of controllers and their capabilities in detail and provides guidance for the development of specifications.

2   The Iowa Energy Center DDC On Line web site at www.DDC-online.org presents 20 different product lines in a generic "ladder" by the classification of the device. The examples used in Figure 2.5 and Figure 2.6 are screen captures from this website.

Interoperability

“The ideal control system is a fully integrated network where all the components share a universally understood communications protocol and similar components from different manufacturers can be directly substituted in a plug and play environment. We used to have this in our control systems; it was called pneumatics.”

J. Jay Santos speaking at the 10th National Conference on Building Commissioning

The advances in control technology brought about by the advent of DDC controls have severely limited interoperability of different control system manufacturers, as described in the preceding quote. Ultimately, DDC technologies well most likely evolve to the level of interoperability achieved by pneumatic components. Until then, there can be significant integration issues that need to be addressed if the equipment from one manufacturer is required to interface with the equipment from another manufacturer. Great strides have been made in this area in recent years through gateways and common languages, like BACnet or LonWorks networks, but true interoperability is still difficult to achieve. Large buildings or buildings located on campuses with central monitoring and control networks may want to include language in their specifications that addresses some or all of the following issues:

Is integration necessary and, if so, what systems should communicate and at what level? This is the key question, and can be as much a matter of Owner preference as a design issue. Since most buildings and the utility systems that serve them must operate as a seamless, integrated whole, a compelling argument can be made for integrating all of the various systems from an operational perspective. You may want to ask yourself the following questions:

1   Should control systems from different vendors be integrated to provide a seamless picture of the site operation at the operator’s workstation?

2   Should the HVAC control system be integrated with the fire alarm system to enhance fire and smoke management functions and response times and to allow “smart” fire detection equipment sensitivities to be integrated with occupancy schedules?

3   Should the HVAC control system be integrated with the security system to allow security detection functions to be integrated with occupancy schedules?

4   Should the HVAC control system be integrated with other facility management systems to allow scheduling functions entered in one system to facilitate preventive maintenance and purchases?

5   Should the HVAC control system and other networked building management functions integrate with the facility or site Intranet to minimize communications costs and maximize the availability of system operating information?

6   How vulnerable does placing the HVAC control system networking function on the site Intranet make it to failures related to network problems and hacking?

7   Will a UL Listing be required for all integrated components associated with a fire or life safety system? If so, can the proposed integration solution provide this?

8   Will customized software be required to integrate multiple vendor packages into a workable system, and if so, how expensive will this be to maintain compared to ongoing revisions to the operating software of all of the systems?

9   Does a manufacturer’s advertised interconnectability translate into interoperability, or will the installation still require significant programming efforts to achieve the desired level of integration?

10 Does the data transfer rate across the interface make the interface a usable tool, or is it so slow or limited that it provides little practical value?

11 Does the integration really lower operating and maintenance costs, or does it increase them due to the need to maintain and be familiar with the multiple vendor tools, products and equipment necessary to perform system specific programming and maintenance?

12 Does the Owner understand that in most cases, “plug and play” integration does not exist in current technology even though the operator interface may make the system appear seamless?

Would direct communications with the control panels on major pieces of equipment be desirable? Most manufacturers are taking advantage of the precision, power and flexibility offered by digital controllers and are incorporating them into the factory supplied control panels for major, technically complex pieces of equipment like chillers or variable speed drives[7]. Frequently, the control packages have access to a wealth of programming and operational data for the equipment served. Making a network level interface with the control package is a cost effective way to provide valuable information that would be far too costly to pick up via discrete sensors. On the other hand, an Owner might be content with picking up critical parameters via discrete connections and checking the other data when necessary via the equipment’s local operator interface.

Should the specification include requirements for compliance with BACNet, LonWorks, or other standard communications protocols? A lot of effort has been directed at developing interoperability standards, and if you need to integrate systems from multiple vendors, properly referencing[8] these standards can provide a robust solution to the integration issue. But, specifying one of the standard communications protocols is not the only option. Many vendors offer their own integration packages to a variety of products and other systems. Systems integration houses are another option if one of the systems that you need to integrate does not support one of the standards. Specifying a standard communications protocol if you don’t really need it may also work against you from a complexity or cost standpoint. A good compromise may be to specify that the system be installed in a manner that makes it physically capable of migrating to a standard like BACNet at some point in the future. Some knowledgeable specification language and shop drawings review will be necessary to ensure that the wiring, network configurations, and object definitions are arranged to allow the targeted standard to be adopted at a later date.

There are no universally correct answers to these integration issues. What matters is that they are discussed with the Owner by the design team and the results of those discussions are reflected in the contract documents.

The following references can provide some guidance in these areas:

·       ASHRAE BACnet website: www.bacnet.org/

·       Echelon website (LonWorks networks): www.echelon.com/

Miscellaneous Components

In situations where the control contractor is furnishing their own wiring system, it may be necessary to reference the materials and methods sections of the electrical specifications to ensure compliance with project standards. As an alternative, the control specification can include the necessary requirements for conduit, wire, cable, junction boxes, and other electrical equipment for the control installation.

Regardless of the approach used, any specialty items necessary for the control installation should be included in the control specifications. Common examples include:

·       Fiber optic cable and/or high speed communications cable

·       Relays, pilot lights and selector switches

·       Power supplies and surge and transient protection.

·       Pneumatic tubing and related fittings and hardware.

·       Control air supply system requirements.

·       Thermometer wells, gauge ports and five valve manifolds

2.4.3.3. Assembly Specifications

Control systems consist of numerous small components that are brought together into a larger assembly. In some instances, including some language in the specification regarding how these assemblies are to be constructed is desirable for quality control.

Including assembly specification requirements in the following areas is often desirable.

·       Control panel installation and fabrication requirements (see sidebar)

·       Enclosure ratings and requirements (NEMA standard, U.L. listing, weather tight, etc.)

·       Shop drawing requirements

·       Component, wire, and cable labeling requirements

·       Wire and tubing routing requirements as necessary to comply with the NEC low voltage wiring classifications (article 725) and minimize noise on sensitive input circuitry

·       Terminal strip requirements

·       Cable bundling and wire-way requirements

·       Flexible wire bundle requirements for cover-mounted equipment

·       Control power disconnect requirements to the extent necessary to comply with NEC

·       Lighting and auxiliary power requirements

·       Shop testing requirements

Some features listed in a control panel specification are not essential for the HVAC equipment to function, but help ensure that the equipment functions as intended by making it easier to perform routine commissioning and maintenance tasks. Documentation of component arrangements using a fabrication drawing and labels will minimize the chances of the wrong component being replaced or removed inadvertently. A neat arrangement within the control panel, facilitated by terminal strips, orderly wiring runs, and a logical arrangement of components will further facilitate troubleshooting efforts. An inexpensive fluorescent lamp triggered by opening the panel door will make a difference when technicians work with small panel components in relatively dark mechanical spaces. A fused receptacle for a laptop computer or other diagnostic tools is helpful when working on a DDC system. This receptacle is not intended for hand tool use, so select a fuse that guards against this use.

2.4.3.4. Operator Work Stations

The operator workstations provided for the system are the interface between the control system and the operators. Including specification requirements that will enhance this interface will be cost effective by improving system utilization and other factors related to day-to-day operations. Consider the following issues when developing operator workstation language.

Software

Is the specific method used to program the system important? Current technology DDC programming approaches fall into two general categories:

·       Line-based programming code. This method technique was originally used by most systems before the ability to easily handle graphics existed. For someone who is accustomed to line-based programming techniques, this approach can be simpler to use and can be more easily represented on text based interfaces. However, it is not as intuitive as graphic based programming and may be more difficult for non-programming oriented individuals to work with.

·       Graphics-based program code. This technique uses symbols with connected attributes to define the program. Frequently, the programs look similar to pneumatic control drawings or programming flow charts, and personnel who are accustomed to thinking in terms of logic flow charts often find this type of programming much easier to work. Editing and troubleshooting this type of code can be a challenge on some systems when there is no way to see the entire program all at once; you have to open individual windows for each element and then remember how they are linked. Other systems allow the entire logic flow to be viewed in a dynamic context.

The programming style can be an important consideration with respect to operator training, ease of use for commissioning, and ongoing maintenance. The desired programming style is a good topic for discussion with the Owner’s operating staff and the commissioning team as you develop your specification. Be aware that specifying one style exclusively can eliminate some vendors from consideration, which may eliminate other equally important features. If you have the luxury of evaluating all bids and selecting your controls supplier based on other factors besides low price, then this feature can be as one of the evaluation factors used to finalize you selection.

Is a graphic interface desirable? Most current technology systems can communicate through text or graphics. Usually, the text-based format is the entry level, low cost approach and will be the standard supplied with your package. Text-based interfaces have the following advantages:

1   No software overhead to support it.

2   No cost required to develop the display screens that will be used by the operators.

3   Requires a lower level, less costly terminal for interface purposes. Usually a “dumb terminal[9]” can be used rather than a portable computer.

4   Requires much less memory to support it.

5   The presentation of information that you see when you are connected to the actual controller will be nearly if not totally identical to what you see when you query the controller from the operator workstation.

6   The number of bytes that must be transmitted to convey the pertinent data is much smaller than for a graphic describing the same information. Thus, the response time will tend to be faster, which can be especially important if the system is being accessed over phone lines.

In contrast, a graphics-based interface will allow the system information to be presented in terms of pictures, system diagrams, and floor plans. The “picture is worth a thousand words” aspect this type of interface can often outweigh any of the advantages associated with a text-based approach, including first cost. The graphics may improve the ease of use of the system. Thus including requirements for graphics interface software can represent a wise investment in terms of the persistence of the design intent and efficiency of your project. However, there are two important things that you need to remember when you do this:

1   The graphic interface is only as good as the graphics it contains. Thus it is important to specify what is required in terms of graphics, a topic that is discussed later in this chapter.

2   For most systems, it will still be necessary to use the text-based interface when connected directly to the controllers. This mode of operation usually occurs for remote access, local troubleshooting and maintenance, and in emergencies when the operator workstation is down or the network is experiencing communications problems. Thus, it is still necessary to provide training for the text based communications approach.

Have software and firmware upgrades through the start-up and warranty year been addressed? Most firmware and software will go through several revision cycles over the course of their use. In most instances, these revisions are triggered by new features, but they can also be made to fix “bugs” that show up in the system after it has been released to the field. In some situations, these “bugs” may not show up immediately and the software and firmware will provide trouble-free service until the triggering situation is set up in the control system. To minimize the potential for the Owner being caught off guard by a problem like this, it can be desirable to include language in the contract documents that requires the contractor to furnish and install all firmware and software upgrades applicable to the system through the end of the warranty year. This doesn’t guarantee a trouble-free future, but it does ensure that the Owner will have the best available version of the product as they exit the warranty cycle and assume full financial responsibility for the system.

Is the ability to remotely access the system desirable? Providing remote control system access via phone modem or the Internet can provide multiple benefits including:

·       Allows operating personnel to respond to and correct deficiencies during non-normal working hours. Often, this can avert major problems and save everyone time and overtime costs.

·       Allows the control contractor to provide better service during the warranty cycle and for subsequent service work. With remote access, the Owner’s favorite technician is often no more than phone call away in an emergency, even if they are on a different site or in a different state.

·       Allows monitoring of a multiple locations from a central point. Owners with multiple, distributed properties can make all of the control systems available to a lead technician at a central location, thereby saving time rather than traveling from site to site.

Security can be a concern if remote access of the system is provided, but most systems have password protection and other features that minimize this risk. If this option is deemed desirable, don’t forget to have the Owner arrange for a dedicated phone line to the control system location. This may mean adding a phone outlet and cable run to your contract drawings.

Is the ability to page on alarm desirable? Anticipatory alarms and their help in avoiding problems are one of the most important benefits provided by a DDC system. However, the alarms will do no good if there is nobody to hear them. Murphy’s law states that most major building problems will occur when there is nobody there to respond. Specifying the ability to dial a pager on alarm can help put the Owner on a more even footing with Murphy & Co.. Typical features include, the ability to dial multiple numbers in sequence or based on alarm priority and the ability to send a text message describing the alarm in addition to a page. Most manufacturers offer remote paging as an option if the correct hardware is also provided. In addition, there are third party systems available that can be driven by I/O points on an existing system that cannot directly support this function.

What other supporting software is required? Specifying that a few additional non-control system software packages be installed and available on the operator workstation can compliment the control system software and increase its utility significantly. The following software packages should be considered:

·       Word processing Providing a word processing package will allow the operators to create electronic log books to document their interactions, adjustments and observations while working with the control and HVAC systems. It will also allow them to create more presentable reports and other useful documents to facilitate their interactions with clients and management. In addition, the operators will have ready reference to the project specifications and other project related documents if they are stored on the hard drive.

·       Spreadsheet software Spreadsheets will allow the operating staff to more readily analyze the wealth of data that the trending capabilities of the DDC system can make available to them. This information can be used for diagnostics as well as reporting purposes.

·       Drawing Software packages like AutoCAD® or Viso® will allow the operators to create their own graphics, view the project construction documents, and even develop their own project documents for minor system modifications. For maximum utility, the package selected should be suitable for generating control system graphics.

·       Management Preventive maintenance programs, purchase order generating programs, and utility tracking software are some example in this category. All of these packages will enhance the user’s ability to take the data from the control system and make it more useful.

·       Reference Many important reference documents are now available electronically. Examples include the ASHRAE Handbooks and the NFPA code books. Having this type of information readily available on a hard drive can enhance the troubleshooting capabilities of the maintenance and operating staff.

Hardware

There are also important hardware considerations associated with the operator workstation.

Is more than one workstation necessary or desirable? A control system of any significant size or complexity will always benefit from a second workstation. The second workstation provides a second means of access in the event the primary workstation fails, as well as a means for the control contractor to perform routine system maintenance without interfering with the day-to-day workstation needs of the operators.

Has a laptop computer been included in the specifications for use in programming and troubleshooting the system in the field? Many DDC problems require observation of the system in the field in addition to observing the information presented at the operator workstation. A laptop computer with the necessary software to interface directly with the controllers allows the operating staff to access the network in the field at any controller location while they are troubleshooting rather than having to communicate with someone at the central workstation to diagnose a problem. Where budgets are limited, the laptop can serve as the second operator workstation in addition to being a portable troubleshooting tool.

Are additional printers desirable? Additional printers can often enhance system utility by allowing alarms to be segregated from other printed system information. A color printer with good resolution provides a means for facility staff to print graphs and other information available in the system for reports and diagnosis.

Has enough temporary and permanent memory been provided? CPU memory is just as important as controller memory, and technology advances have made it affordable. Providing plenty of RAM will help the system graphics load and update quickly, which is especially important when network traffic rates are high. Providing plenty of hard drive space will allow the Owner to maintain an archive of trend and utility data which will aid in diagnostics and maintaining peak operating efficiency.

Has a means of backing up the system been provided? The fully commissioned control system represents a significant investment in manpower. Much of this investment is reflected in the data entry associated with programming the system and in the modifications made to the initial database and tuning parameters as the system is brought on-line and operated. There is also an investment in the operating software to customize it to the needs of the project and the tastes of the operating staff. The failure of a hard drive or its controller can often eradicate this investment in a heartbeat. Thus, it is essential that the system be provided with a reliable, easy means to perform back-ups. The control contractor should develop this back-up strategy, then train the Owner in its use. The back-up strategy requirements should include documentation for the frequency of back-up, the number of copies made, and where they will be stored.

Graphics

Graphic operator interface programs are becoming more common as the standard operating package provided on many systems. When properly developed, a good graphics package can make the control system more user-friendly to the operators. To achieve this, the specifications need to include details about the graphic requirements beyond simply stating that the system should have a graphic operator interface with a graphic for each HVAC system.

In a large facility with multiple systems, finding the graphic you need can be overwhelming. However using an intuitive strategy like a nested graphics structure that leads the operator to the correct graphic can solve this problem. For example:

·       An alarm condition can either trigger a graphic or generate a message that directs the operator to the correct graphic. The graphics themselves can contain information on how to address the alarm as well as presenting the system in alarm. However, be aware that some care needs to be exercised when using graphical alarms. An emergency that generates multiple alarms can crash the system or make it inaccessible to operators during a crucial time period by overwhelming the workstation with graphic requests.

·       The opening screen can start with an overview that can be used to further penetrate the database in the area of interest. For example, the system could start by presenting the operator with a graphic of the site plan. Clicking on the building of interest would open a floor plan of the building. Clicking on the equipment room area would open up a large scale floor plan with the equipment locations shown. Clicking on the piece of equipment of interest might open a graphic that is an actual picture of the machine in question with pertinent data at the appropriate locations. Buttons on this graphic might link to a schematic of the system, pictures and schematics of related equipment and support systems, and screens that allow set points to be adjusted. When using this nested approach, it is important to make sure each graphic has a button that will take you back to the screen you came from.

·       Tabular Graphics Projects with a number of similar pieces of equipment may benefit from including a tabular style graphic in the project requirements. This type of graphic is similar to the equipment schedule on the drawings but contains dynamic updates of key operating parameters. Such an approach is useful during some testing processes as well as during emergencies when the operators need to see the big picture. It is often convenient to include buttons in the table for each system that take you to the detailed graphics associated with the system. It may also be desirable to have buttons that trigger emergency response processes, like a load shedding routine.

To achieve the utility described in the preceding paragraphs, it is necessary to include the requirements for a typical system graphic in the project specifications. In addition to the features listed above, the following issues should be addressed:

·       Dynamic information will provide the greatest utility when working with graphics. Maximum permissible update times should be specified to ensure that the contractor provides a system architecture and graphic structure that provides timely information.

·       Ideally, the schematics on the graphics should match the ones on the drawings (assuming they are well developed) to make it easier for the commissioning team and operating crew to correlate the control system information with the drawings. The schematics should match the actual installed field connections including the order that systems connect to headers, control valve locations (supply side vs. return side), and sensor locations (see the side bar below).

·       The specification should include language that requires the data in the graphic to be legible and forces the contractor to additional graphics if one graphic cannot contain all of the desired information in a legible format.

·       It may be helpful to include guidance regarding the desired connectivity from one graphic to related graphics when a nested structure is employed.

·       Providing tables, switches, knobs, dials, or sliders to allow set points, start-stop commands and tuning parameters by be set via a graphic (by someone with an appropriate access level) can add utility to the system. Compared this to locating the required parameter in the point database or in the program code and to make the adjustment.

Some designers have found that a standard detail depicting the graphic format they desire is a convenient method to convey much of this information.

Having graphic schematics that match the actual installed system may seem obvious, but there are a surprising number of projects where the graphics are not correct. On a recent project, the contract drawings contained two different diagrams of the heating water system. The control schematic in the shop drawings, presented a third interpretation of the system. None of the three schematic drawings actually matched the installed piping configuration. Since the system contained 8 boilers and 11 pumps arranged in a variable flow, primary-secondary configuration, the exact order of the connections has a significant impact on the temperatures produced when the water from the various elements mixed. Interpreting system data presented on an incorrect schematic could lead operators to incorrect conclusions.

2.4.3.5. Installation Requirements

Specifying installation requirements for items that were furnished by the control contractor but installed by other trades have been discussed. It is desirable to also provide some language in the specification to help ensure that the equipment installed directly by the control contractor meets the necessary standards. Items that may merit consideration include:

·       Rough-in Requirements for Room and Outdoor Condition Sensors Room sensors may need to meet ADA or owner requirements with which the control contractor may not be familiar. Or, there may be architectural considerations or furniture layout requirements that would impact the sensors ability to perform. It is not unusual for the control contractor to be off-site early in the construction process since most of their installation cannot occur until the systems are in place. However, sensors need to be located in masonry walls, lath and plaster walls, cast in place concrete walls, or similar locations, getting them in after the fact can be difficult if not impossible. While being familiar with all of these contingencies will technically be a part of the control contractors obligation when they sign there contract, providing a few sentences of guidance in the control section of the specification to alert them to the contingencies can help avoid problems during construction for everyone.

Outdoor air sensors can be of particular concern since their performance is critical to the operation of the system. If you ask a room full of engineers and control technicians where the sensor should be installed, you will probably get more than one answer. Thus, it is usually in everyone’s best interest for the designer specify an outdoor air sensor location on the contract documents, review this with everyone, and reach an agreement prior to actually installing the sensor.

·       Sensing Element Installation Requirements While in the general case, the control contractor can be relied upon to properly mount the systems sensors, providing some details on the drawing can ensure that your design intent is realized for specialized applications (Figure 2.7). Installation details can also help support the control contractor in situations where another trade is involved with the installation (Figure 2.8).

Figure 2.7 Typical Installation Detail for a Specialized Application

AutoCAD® users can double click on this figure to open it as a drawing.

Figure 2.8 Typical Installation Detail for Multiple Trades

AutoCAD® users can double click on this figure to open it as a drawing.

·       Transmitter Location Requirements A transmitter converts a low grade, noise susceptible, non-standard signal a higher grade, industry standard, noise immune signal so that it can be passed over distance without degrading. In other words, it is more than just a signal converter and the project documents should ensure that transmitters are installed in a manner that fully utilize the benefits they provide. This is discussed in more detail in the Functional Testing Guide, Chapter 5: Supplemental Information, Section 5.6.4.4.

·       Five Valve Manifold Requirements Differential pressure sensors can be subjected to decalibration or even destroyed if their sensing elements are exposed to full line pressures. Valve manifolds with equalizing connections can avert this and should be specified where applicable. This topic is discussed in greater detail in Chapter 3, Section 3.5.3.1: High Pressure Aplications.

·       Pneumatic Tubing Related to Smoke Control The tubing serving smoke control dampers may have special installation requirements in certain jurisdictions including the need to run all copper to minimize the potential for a tubing failure during the early stages of a fire. Such requirements should be clearly stated in the control specifications.

·       Tubing Used to Connect the Sensing Ports of Transmitters that Monitor Low Pressures to the Location Where the Pressure is Sensed The long tubing runs frequently used bring low air pressure signals to a static pressure transmitter can cause problems related to their length and leakage. These issues are discussed in detail in the Functional Testing Guide, Chapter 5: Supplemental Information, Section 5.6.4.4. Specification language or installation details should clearly direct the contractor with regard to your expectations for this type of application.

2.4.4. Floor Plans

Developing floor plans for the control system is beyond what most designers typically include in their process. However, those who can afford to go this extra step, will see benefits during the bidding, construction, and operation phases of the project that help ensure a quality product. The ideal control drawing background will include both the mechanical and electrical systems since the control system generally must interface with both. Developing this type of background is fairly easy due to the external reference capabilities of CAD programs. Another option that may further reduce the effort is to include the information on the project’s mechanical or electrical sheets rather than generating a separate set. The illustration in Figure 2.9 was extracted from a control drawing set to illustrate control system floor plans.

Figure 2.9 Mechanical Room Drawing with Control Information

The background for this floor plan was generated quickly in AutoCAD® by external references to the mechanical and electrical drawings. The control information and points were then inserted. The points have AutoCAD® attributes associated with that are entered when they are inserted on the drawing. An automated AutoCAD® procedure can be used to extract this information to generate a point list. Most of the attributes are invisible, but the point number is displayed along with the point symbol. The coding of the point number reveals useful information about the point. For instance, the number circled in red is coded to indicate the location (the 01for the central plant), the sensor type (Immersion Temperature, High precision), the number (003 for the third point of its type), and is furnished under the base bid (BB). AutoCAD® users can double click on this figure to open it as a drawing.

From the designer’s perspective, developing the floor plans provides one last cross check for coordination issues, especially if the control drawings are developed as a separate set using references to both mechanical and electrical sheets. Thus at least a portion of the development costs associated with the set can often be written off against coordination checks that are normally done as a part of any design process.

Providing floor plans for the bidding phase allows the contractors to better understand and coordinate their work. Having floor plans with important control system information on them allows the controls contractor to devote their time during the bidding window to tailoring their equipment and system to the requirements of the project rather than to developing a basis for obtaining a wiring price. As a result, the controls contractor will generally have a higher level of comfort with their bid and will feel comfortable bidding at lower profit margins since the enhanced understanding of the project can reduce their risk. The floor plan information also helps ensure that the bids from different vendors reflect equivalent systems.

The benefits of having floor plans as a part of the construction drawing and record set are more obvious. The information will help contractors interface with control work, help the owner and operators become familiar with the building controls, help the designers communicate their intended design, and help the commissioning provider put together commissioning specifications before all of the control drawings are available. Items that might be included in the floor plans are:

·       Point locations

·       Major cable routes

·       Power sources and wiring requirements

·       Controller and auxiliary panel locations

·       Operator work station locations

2.4.5. Standard Details

Standard details for the control system allow the designer to ensure that the control contractors meet the design intent. Obviously, developing these details will take some initial effort, but once they are developed, they can be used again and again with minor modifications as required to meet the project’s specific needs.

Given the wiring intensive nature of DDC systems, detailing wiring requirements can provide important guidance for the construction team. At first, this may sound like a time consuming and difficult area to detail due to the many manufacturers and systems available in the industry and the perceived complexity of the technology. However, if the focus of the effort is to show intent rather than system-specific information, developing the details can be a fairly simple, one-time process, that will improve the quality of the project when included in the designer’s construction documents. Often, the necessary details can be developed from information included in the control shop drawings associated with a past project.

The benefits of developing these details are similar to those achieved by providing floor plans. The contractor can more easily develop a better price since intent is conveyed to all parties in the process. Specification language and/or notes can clarify that the details are intended to provide guidance during the bidding process and that the contractor is still required to develop system and project-specific wiring information as a part of the control submittal package. Typical wiring details might include:

·       Control panel power requirements.

·       Control panel arrangements.

·       Input and output wiring for the various input and output types.

·       Starter interlock wiring, including permissive type interlocks.

·       Variable speed drive interlock wiring.

Other areas where detailing can provide significant benefit include.

·       Sensor installation requirements

·       Typical graphic requirements

·       Typical damper and valve requirements

·       Control panel requirements

Figure 2.10 and Figure 2.11 are examples of some details of this type.

Figure 2.10 Typical Control Panel Detail

This detail is from the same project as the floor plan that was used in Figure 2.11. AutoCAD® users can double click on this figure to open it as a drawing.

Figure 2.11 Typical Analog Input Wiring Detail

This wiring detail illustrates an analog input using several of the specialty terminal strips discussed here and in Chapter 3: Control and Monitoring Points.

The specialized terminal strips shown in the typical diagram in Figure 2.11 bear special consideration and discussion. While quite common in the industrial and process control systems, the technology represented by these terminals has yet to penetrate the HVAC industry. This is partly because designers are simply not familiar with it, and partly because it is seen as an unnecessary added cost. From a commissioning and operations perspective, the functional benefit from these devices can outweigh the minor costs associated with their installation. From a lifecycle perspective, the first costs are often saved during the start-up and first year of operation.

Early recognition of the advantages of centralized monitoring and control caused a Midwestern University to develop and install such a system starting in the early 1980’s. The system started out as a supervisory monitoring and control system for critical buildings and evolved as time and budget allowed. From the beginning, standard input and output strategies like 4-20 ma, platinum RTDs, and 3-15 psi had been used, and field wiring had been terminated using specialty terminal strips similar to those described in this chapter. In the late 1990’s, a project was undertaken to replace the original programmable controller and IBM Series 1 mainframe technology with state-of-the-art DDC technology, and make DDC the control standard for all future projects. Because the original I/O wiring and sensors could be re-used, and the flexibility provided by the existing terminal strips reduced changeover and check-out time, project installation costs were 40 to 60% lower than typically expected with a control system replacement.

The benefits of terminal strips include:

1   Identification of wiring to speed up maintenance and troubleshooting and minimize confusion and mistakes.

2   On projects where the sensors and their wiring system are installed by an independent contractor, the terminal strips can be used to define the contract boundary between the wiring contractor and the control contractor.

3   Input signal quality persistence can be improved in some applications by minimizing problems with poor connections that can occur when solid conductors like the leads from scaling resistors are clamped under the same terminal as a stranded wire (see Chapter 3, Section 3.3.4.3 Scaling resistors).

4   Easier troubleshooting because the terminal strips provide a standard wiring arrangement, thus a standard troubleshooting procedure can be developed. Often, this will allow the initial troubleshooting effort and some repairs to be accomplished by a lower level technician, which reduces costs and frees up other staff members. In the long-term, terminal strips make future controller upgrades easier to accomplish. Most of the technical evolution that has occurred in the DDC field occurred at the controller, network, operator work station, and sensors. Future evolution will undoubtedly continue in these areas. Despite these changes, the wiring requirements associated with the inputs and outputs seen on most HVAC systems have changed very little, especially in situations where standards like 4-20 ma, 1-5 vdc, etc. have been employed. Thus, while technological improvements may result in Owners upgrading some of the hardware in their systems, the wiring system can often be retained and re-used. Terminal strips can make controller and sensor upgrades easy to accommodate, further reducing the costs associated with the improvements. The case study in the sidebar illustrates this advantage.

Figure 2.12 takes a close look at typical specialized terminal strips.

Figure 2.12 Typical specialized terminal strip installation

The area inside the circle on the left is shown in the right picture. These terminals ground the shields for the I/O cables (green and yellow terminals), allow field wiring to be isolated for testing (orange terminals) and scale analog inputs (bottom terminal).

2.5. Putting These Concepts to Work

By providing some or all of the control system design features discussed in this chapter, a designer can improve the quality of their projects as well as distinguish themselves from other consultants to a savvy Owner, thereby improving profit margins and client relationships. The information and tools provided in this chapter will help put these concepts to work. The remaining chapters in the Control System Design Guide serve the following functions:

Chapter 3: Selection and Installation of Control and Monitoring Points Chapter 3 describes sensor technologies, sensor accuracy, installation issues, sensor selection and installation guidelines, and the point interface to the building automation system. The sources of error in a measurement, from the sensor to the control system workstation are described in detail.

Chapter 4: System Configurations Chapter 4 includes sample point lists for many common system configurations. These lists can become the starting points for future projects.

Many of the fundamental building blocks required for the control system design process are available within the Control System Design Guide. One of the best ways to get started on the process is to choose a relatively simple project for the initial effort, perhaps one that has only one or two air handling systems that serve a few zones in a standard configuration. The details and specifications developed in this initial project can then serve as stepping stones to larger, more complex projects in a path to improved control system design and better functioning, more efficient HVAC systems.



[1]   For example, the point list might clarify that the sensor shown on the system diagram at the discharge of a cooling coil with face and bypass dampers must be installed in a manner that ensures it will see only cooling coil discharge air, not the mix of cooling coil discharge and bypass air, thus ensuring the functionality of the dehumidification sequence.

[2]   The suffix would only be used during the bidding cycle and then dropped when the system was installed and programmed.

[3]   AutoCAD® blocks coupled with visible and hidden attributes can be used to automate this process during the development of design drawings. The point blocks can be inserted where appropriate on the floor plans or system schematics and the attributes defining the point list information entered as a part of the insertion process. Then a LISP routine can be used to scan the drawings, extract the point information from the point block attributes and generate the point list. A similar approach can be used to automatically generate valve and damper schedules.

[4]   A variation on the latter design approach are controllers with fixed point counts that can be adapted to a variety of input and output signals via termination techniques and software parameters that are set by the installing technician. Some are so flexible that the points can serve as inputs or outputs.

[5]   Some Facilities Engineering groups consider this feature to be important enough that they make it a mandatory part of their system requirements. If a manufacturer does not offer this feature as a standard built-in component on their circuit boards, then they must provide the function in an auxiliary field panel.

[6]   Copying and modifying logic from a similar project can offer some benefits in terms of reducing programming costs. Once a program is developed and debugged, it can be applied to similar systems on a current project to minimize development and start-up costs.

[7]   See Direct Network Connection of Variable Speed Drives by Thomas Hartman in the March 2003 issue of Heating, Piping, and Air Conditioning for a detailed discussion of this topic as it relates to variable speed drives. The article can be downloaded from www.hpac.com/.

[8]   It is important to understand that simply referencing the standard does not ensure integration and interoperability. The contract documents need to include the necessary language to define the specific level of interface you are trying to achieve, including object definitions, wiring and networking standards, etc. In many cases, simply specifying the standard and not covering the details associated with it can cause more problems than it solves.

[9]   A dumb terminal is essentially a monitor and has very little processing power. A common example is the VT100 technology used for modem-based communications. In contrast, most system graphics are actually built up from numerous system objects and the device that displays them needs to be running software that can put the graphic together with access to the necessary files. Memory limitations in most controllers prevent the file structure and objects from being stored there. Typically the graphics are assembled from objects stored on the operator workstation hard drive and then populated with data retrieved from the field devices and controllers. This process is usually a memory and processing speed intensive.