The ultimate goal of the commissioning process is the reliably integrated control and operation of a building and its systems.. Success comes as a result of an orderly, methodical process that is integral with the design and construction of the project from its start and is embraced by all involved parties. Ensuring the persistence of those benefits involves an ongoing commitment from the building’s management and operating team for the life of the facility. Achieving success can be challenging due to the complex interactions that occur simultaneously among the systems, system components, the building envelope, the building’s external environment, and the building’s occupants.
This chapter focuses on these interactions. While using the chapter, it is important to remember that just as the proper and integrated operation of an HVAC system relies heavily on the thoroughness of the commissioning process and the robustness of its individual components, the content of this chapter draws heavily from the content of the component-focused sections of the Functional Testing Guide.
The concepts discussed in this chapter are applicable to most system-level commissioning processes, regardless of the system type or size. Pumps must be integrated with the piping system and the energy-conversion equipment they serve. Lighting controllers must be integrated with the building schedules, the occupants’ needs, and the effects of day lighting.
In fact, what constitutes a system can be broadly defined. At one end of the spectrum, a simple preheat coil and its associated control valve, piping, and controller can be seen as a “system” in that successful operation can only be achieved if all of the individual components are made to function in harmony. But at the other end of the spectrum, the preheat coil is only one subassembly of a larger and more complex air handling system. In addition, the air handling system is served by other complex systems like cooling, heating, and pumping systems, each of which are comprised of several components and subassemblies of their own. All of the components, subassemblies, and systems must work together for overall success to be achieved. The following definitions of components, subassemblies, and systems are used throughout the document.
· Components/Subassemblies The term “component/subassembly” refers to individual pieces of equipment that will be joined together to make up a total “system.” Examples of components include coils, humidifiers, fans, pumps, economizer dampers, chillers, cooling towers, and boilers. Each of these components typically includes several subassemblies. For example, the subassemblies associated with a cooling tower may include tower fan VFD(s), control valves, DDC controllers, and sensors used in the control loop.
· Systems The term “system” refers to the aggregation of individual components and/or subassemblies into a single system. For example, an air handling system may contain the following components: pre-heat coil, heating coil, cooling coil, humidifier, supply and return fans, and outdoor, return, and exhaust air dampers. Similarly, a pumping system will typically consist of multiple pieces of equipment and subassemblies like pumps, control valves, sensors, VFD’s, and filtration components. Again, the individual component/subassembly tests will be performed prior to evaluating integration and control of the complete “system.”
Finally, from the building occupants’ perspective, a comfortable, controlled environment will only be ensured if all of these components, subassemblies, and systems can work together in harmony with other systems including the local environment and the building functions and envelope. Commissioning HVAC systems for proper integration and controls generally follows the “bottoms-up” testing approach as outlined below.
· Supporting utility system testing. The utility (electric power, natural gas, water, or central plant) serving all components, subassemblies, and systems must be available. For example, commissioning includes verifying that supply voltage and/or pressure meet manufacturer’s requirements of the respective components and systems.
· Component/subassembly testing. The specific control strategy associated with a component/subassembly is verified to function correctly before it is tested as a total “system.” For example, it would be impossible to verify the discharge air temperature control strategy for an air handler system until operation of the cooling coil and cooling system component verification tests are complete.
· System-level testing. Once individual component/subassembly operations are verified to be correct and stable, then an entire system can be tested. For example, all of the components of a cooling system must interact together to form a cohesive system.
· Multiple systems and building interaction. Individual systems must work in unison with each other and with the overall building in order to provide a safe and comfortable environment for the occupants. For example, both the cooling and pumping systems have a major impact on the discharge air temperature from an air handling unit. Control of the water temperature and flow rate from the respective system must be stable in order for the discharge air temperature control strategy to function properly. Conversely, instability in the economizer damper or cooling coil subassemblies in an air handler system can cause instability in the respective cooling and pumping loop control strategies. Proper economizer damper control also impacts building pressure. Instability in the economizer control strategy can result in poor indoor environmental quality, from infiltration of outdoor air, moisture migration, doors that are hard to open, or “whistling” noises between zones with significant pressure gradients. All systems must work together to provide stable control.
· Real time operation. The final step in the bottoms-up testing approach requires the monitoring of system performance over time. System integration testing for most projects occurs during off-peak conditions or under false loads. The test conditions may not reveal integration and control problems that could manifest under natural atmospheric and building operating conditions. Systems that were stable during initial testing may become unstable when seasons change or as building loads vary. Trend analysis is the most typical method for monitoring system operation to verify proper performance and troubleshoot problems.
Delivering this whole-building, integrated level of performance is the ultimate goal of most commissioning processes, but it can be difficult to achieve. This chapter will impart fundamental concepts fortified with wisdom gained from field experience to make integration easier. The chapter discusses the fundamental principles of integrated, interoperable building systems and instructs on how to test and fine-tune building systems under diverse operating conditions.
HVAC systems are complex, integrated machines whose components must interact successfully under a wide array of operating conditions. Instabilities in one small element of a system can cascade into system-wide or even building-wide problems. Further complicating the situation, the time constants and other parameters associated with system and control loop stability vary with the seasons and the age of the equipment. A system that is stable in one operating mode may exhibit erratic performance when the seasons change or the fouling of a heat transfer surface shifts performance outside of the original operating envelope. The true test of whether or not an HVAC system is performing according to its design intent is how well the various components are integrated with each other.
The bottom line is that the successful interactive operation of any system goes hand-in-hand with successful control system design and integration. The following criteria for control systems will make integration easier:
· Select and install control components to promote integrated performance.
To explain by example, theserving an economizer cycle and the damper and relative to each other can have a critical impact on the air handling system’s performance.
· Tune control loops and components for stable performance.
For most HVAC systems, the output of one control loop becomes the input to another, whose output may then be the input to a third. In fact, the response of a control process at the end of the system frequently becomes an input to the process at its beginning. The control processes in AHU1 are an example of control loop interaction, as illustrated in.
The blue lines represent the discharge-temperature control loop and the impact of the cooling coil and economizer dampers that it controls. The green lines represent the mixed-air- low-limit temperature control loop and the impact of the economizer dampers it limits. The red lines represent the preheat discharge temperature control loop and the impact of the preheat coil it controls.
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is a good illustration of how complex interactions inside buildings can be. The actions of all three control loops in the diagram have the potential to influence not only their primary control element, but also the conditions entering the control processes downstream. As a result, instability in one loop can have ripple effects on the others.
illustrates what happens when these interactions get out of control. In this particular instance, the instability in the temperature control loops captured by the trend also triggered instability in the air handling system flow control loops, which are not illustrated. In turn, this resulted in pressurization of the outdoor air intake plenum by return air, which impacted the outdoor air temperature sensor. This created a false outdoor air temperature spike that triggered multiple start-ups of the small, unattended chilled water plant! And, despite all of the “activity,” the occupants and owner were unaware of the issue because their primary indicator of satisfactory performance was comfort, which, as can be seen from the zone-average temperature and return air temperature trends, were solidly at the desired set point.
Detecting and eliminating problems of this type is crucial to ensuring optimum efficiency and equipment life. Preventing them is even better. The remainder of this chapter will be devoted to a discussion of procedures and techniques that can be used to minimize exposure to integration problems from the start and detect and resolve them when they do occur.
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An air handling system associated with a renovation project in a hypothetical building will be used throughout the chapter to aid in the discussions. Selecting an air handling system as a framework for our discussion allows integration issues to be explored. The system depends upon the proper functioning of the supporting utility systems and its components to deliver the desired level of performance. Proper control of conditioned spaces relies on the air handling system to accommodate the variable impacts of the occupants, the envelope, and the local environment.
The system in this example is a VAV-reheat system serving a student center and administration building on Midwestern college campus. The new system was installed as a part of a renovation project that occurred over the fall, winter, and spring months and serves a number of functions including offices, a museum, and the student union and lounge. The project occurred in a number of phases to minimize disruption to the student activities during the construction process. The final stages of the commissioning process occurred in June, July, and August to allow the renovated areas to be ready for start of fall semester. The HVAC systems for the renovated areas, which featured packaged systems and rooftop equipment served by local chilled water and hot-water-heating systems, were upgraded and connected to the campus centralized chilled water and hot-water plant. HVAC systems that served areas not affected by the renovation were also connected to the central plant.
The building and HVAC system will be referred to as the “student center” and “AHU1,” respectively, for the remainder of the chapter. The details of the renovation and new air handling system, including a description of the building and loads it serves, a system diagram, overview of operating sequences, design specifications, and the mechanical room plan have been included for reference in.