The commissioning and operation of HVAC and control systems are interdependent and inescapably linked with the successful performance of a building. The design and installation of these systems must consider the interaction of components with each other and with the building, its occupants, and the environment. Bringing an integration perspective to the process early in the design phase is essential for success to be achieved, as discussed in
Commissioning and operation build on the integrated-design foundation by testing, observing, and tuning the response of individual components and control loops as they interact with each other and the response characteristics of the systems, the building, the occupants, and the environment. These activities are best initiated immediately after design as the project moves into construction, and they must persist for the life of the building. Such an approach will go a long way towards ensuring that the design intent is met at the completion of the project and is adapted to the changing requirements imposed on the building by the real-time operating environment.
As shown inand , most HVAC processes are complex strings or cascaded control loops, where the output of one becomes the input to another. For most systems, the resulting interactions are not necessarily constrained to processes controlling similar parameters. The following example shows how changes in one control loop can cascade through to multiple systems.
Consider the following discussion regarding AHU1’s control processes. (It will be helpful to refer to the system diagram provided in Figure A-1 in Appendix 1 as you read this section.) As the discharge temperature control process modulates the economizer dampers, its actions can impact the supply fan flow control process. This occurs because the movement of the dampers will change the flows and pressures in the mixed air plenum, even if they are properly sized. In turn, the resulting changes in the supply fan speed will change the system air flow and the pressure drops generated by that airflow, which will have the following results:
· Economizer performance. Changes in supply fan speed will change air flows in the system, including the quantity of outdoor air and return air. Unless both the economizer and return dampers are sized for perfectly linear characteristics, variations in supply air flow may result in unpredictable amounts of air flowing through the economizer and/or return air dampers. Large variations in outdoor or return air may result in unstable operation of the discharge-temperature control loop and economizer performance.
· Pressure relationships between spaces. Changes in differential pressures between spaces cause a ripple effect downstream of the supply fan. A review of the sequences of operation for our example (Table A-1 in Appendix 1) reveals that these processes are passively controlled by design and balancing – that is, no control system is directly responsible for pressure relationships between spaces.
· Building pressure relative to atmosphere. Changes in the indoor/outdoor pressure difference cause another ripple effect resulting from movement of the economizer damper. Economizer dampers change outdoor air and return air flow rates and supply fan speed, both of which impact how air is distributed through the system. In contrast to the preceding bullet, these effects cascade into actively controlled HVAC processes, such as the building static-pressure control loop.
· Return fan performance. The return fan performance is affected by a change in building static pressure. As can be seen from Table A-1, a change in building static pressure can change the position of the relief dampers. If the relief dampers’ position changes, the pressure in the return fan discharge plenum also will change. When this happens, Table A-1 shows that the return fan’s discharge-static-pressure control loop will change the return fan speed to compensate.
· Air flow through the return damper. This is the direct result of the change in return fan speed and another feedback into the economizer process, which originated the disturbance. The change in return air flow will change the mixed-air temperature and can also influence the pressure in the mixed-air plenum, all of which feeds back into the various control loops as described in the preceding bullets.
· Performance of other air handling systems in the building. The building pressure relative to atmospheric pressure will directly or indirectly impact the performance of all air handling systems serving the facility. The impact will generally be negligible for simple systems without economizer processes; for example, fan coil units. On the other hand, if there are other major economizer-equipped, air handling systems that incorporate control processes similar to those described for AHU1, the impact can be significant.
· Performance of the central plant. The flow and temperature variations that occur in AHU1 directly impact the performance of its cooling and heating coils, affecting both the load that must be handled and heat transfer characteristics of the fins and tubes. Obviously, these changes will impact the loads seen by the central plants that serve the heating-hot-water and chilled water systems serving the coils.
· Electrical distribution system. Although the dynamics of the electrical system are less obvious than those associated with the mechanical systems, they still exist. The varying loads on the supply and return fans will change the current flow through the motors, drives, and distribution system. Additionally, the variable speed drives controlling the fan motors will generate harmonics on the power system and the nature of the harmonics will vary with the load. Generally, these effects will not be considered by the mechanical system commissioning process. However, they would be integral to the commissioning of the electrical distribution system, especially the coordinated fault detection and control system and the assessment of current flow in the neutral and grounding systems. Interestingly enough, if the building was served by an on-site generation system, the electrical loads would eventually ripple back to mechanical issues. This “re-surfacing” would occur at the cogeneration plant in terms of the performance of the prime movers generating power, the heat they rejected, and the heat balance of the plant (a critical factor in cogeneration plant efficiency).
The complexities associated with integration will typically extend beyond a building’s HVAC systems and their control loops and into operating considerations such as hourly changes in occupancy, seasonal variations, and equipment wear. Many of these problems are not easy to detect, especially when occupant comfort is used as the primary indicator of satisfactory performance. The following section lists these operating considerations, using the case study building as an example:
· Building use patterns impact the integrated interaction and response of the systems serving the building.
In the Student Center, the highly variable traffic patterns through the lobby doors could trigger instability in the system. From Table A-1, it can be seen that AHU1 controls the relief system for the building to ensure a positive lobby pressure. In addition, AHU1 has the ability to bring in additional outdoor air if the relief dampers are closed and the lobby pressure is not at set point. Both of these features have been implemented to ensure a warm lobby during the winter months. All other things being equal, the primary reason the lobby pressure would fluctuate is the opening and closing of the lobby door. When the door is open, it represents a major breech of the building envelope and the lobby pressure will drop. The building static pressure control system would respond by closing the relief dampers. The change in relief damper position would change the return fan discharge static pressure and result in a reduction in return air flow, making more air available to exit through the open door, thus maintaining the lobby pressure. When the door closes, the opposite sequence of events would ensue.
The bottom line is that the opening and closing of the door generates an input to the building static pressure control system, which triggers a response. If the door opens and closes quickly when one person goes through it, the triggering input will be different from that which would occur if the door was held open due to a steady stream of traffic. A condition between these two extremes would generate a string of triggering pulses in short succession.
Now let’s think about how the Student Center is used. During periods of low activity, the door might only open briefly one or two times an hour. The resulting pressure changes would be brief in duration and small in magnitude. The response would only require a modest repositioning of the relief dampers to ensure a positive lobby pressure. However, when there is a lot of activity in the student lounge, the door might open many times per minute or stand wide open for minutes at a time. The resulting pressure fluctuation in the lobby would be significantly different from that generated from the periods of low activity, when the door is opened or closed infrequently. A system that was tuned to be stable at one traffic rate may exhibit instability at the other.
· The ambient environment will impact the response of the building and its HVAC equipment.
This seems obvious, and in many ways, it is. However, there can be some subtle effects that lead to problems down the road if that potential is not recognized. Returning to the lobby pressurization example discussed in the preceding bullet, consider the following: If the ambient temperatures were at or near the required discharge temperature and the building was at peak occupancy, the position of the economizer dampers would be at or nearly at 100% outdoor air, the stack effect would be minimal, and the supply fan speed would be relatively high. In contrast, if the outdoor temperatures were at the winter design condition and the museum was the only portion of the building that was occupied, the economizer dampers would be at or near the minimum outdoor air position, the stack effect would be more pronounced, and the supply fan speed would be significantly below full-speed. Under these conditions, the performance of the economizer in terms of mixing and linearity would be quite different from those associated with the warmer, full occupancy day. In addition, the impact of the lobby door opening on a building-pressure would be different than what would occur on the warmer day. Thus, it can be concluded that a system that was tuned and stable under one condition may not provide satisfactory performance under the other due to the different performance and response characteristics associated with the different conditions.
· Time affects integration and interactions.
The rate at which change occurs in a process and in the loads the process serves change with time. This time-related effect was demonstrated by the rate at which the Student Center lobby door was opened and closed, as discussed earlier. The more subtle impact of time occurs over the long term, typically manifested as wear or other age-related effects. Wear includes degradation of seals and linkages, actuator failures, and the accumulation of dirt. For example, if an actuator on the economizer dampers in the Student Center AHU1 were to fail and it was not replaced with an identical unit, or if the linkage arrangement was different than the original installation, the relationship between actuator stroke and damper blade position could be changed. As a result, economizer related problems could surface in a system that had performed satisfactorily for years. Or, the Student Center lobby door seals could wear or the door closer speed change due to wear. As a result, the lobby leakage rate or the building pressure response to a door opening could change triggering pressurization related problems. At the extremely subtle end of the scale, a layer of dirt could accumulate on the large, high volume, low static pressure, slow turning, low horsepower return fan wheel, changing its moment of inertia and starting torque requirement. As a result, a fan that had been starting reliably for years could suddenly begin to trip overloads at start-up.
· Systems can mask problems when viewed from the perspective of the occupants of the space.
In this example illustrated in, instability in the hot water valve loop triggered problems with the economizer control loop, the fan speed control loop, the relief damper control loop, and building wide functions triggered by outdoor air temperature. During the trend window, the most common indicator of acceptability - occupant comfort - was satisfactory as can be seen from the stable zone temperature. The problems documented by the trending had potentially been occurring for weeks but had gone unnoticed by the building occupants because their primary indicator of functionality, space temperature, was fine. The thermal inertia of the building and the system interactions completely masked the problem from an occupant’s perspective.
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illustrates another common example where an HVAC reheat process combines with the thermal inertia of the system and building to mask dysfunctional machinery from an occupant’s perspective. More than energy is being wasted. The 10 or so cycles per hour translate to 87,600 cycles per year; well beyond the design life for even the highest quality commercial valve or damper actuator.
To summarize what has been discussed so far:
· HVAC processes and their control systems are highly interactive with each other and with the building envelope, its occupants, and the environment.
· Components, control function, and systems that perform satisfactorily when tested on their own can become unstable when they interact with each other, the building, its occupants, and the local environment.
· Components, control functions, and systems that perform satisfactorily when tested on their own can become unstable as the impact of time takes its toll.
· A dysfunctional system can appear to be working fairly well if the acceptance criteria is occupant comfort.
The good news is that commissioning, in general, and functional testing, in particular, provide ways to address these issues. Specifically:
· Design-phase commissioning provides a way to identify and avoid problems.
An ounce of prevention is worth a pound of cure. Design-phase commissioning provides that ounce of prevention by bringing an integration and operations and maintenance perspective to the project from the start. The commissioning process benefits from this early participation because having the commissioning provider participate in the design phase promotes documentation of design intent. Participation in the design phase allows the provider to interact with key architects of the design intent, such as the owner and the design team. These topics are discussed in greater detail in .
Also, early knowledge of design intent allows the commissioning plan and test plan to focus on areas that will yield the “biggest bang for the buck.” The knowledge gained during design phase will help the commissioning provider understand the critical performance parameters for the systems to be commissioned and where the weak points are. As a result, the commissioning plan, specifications, test plan, and budgets can be tailored to meet the project needs of all parties.
· Commissioning provider participation during the construction phase helps to ensure that the installed systems reflect the design intent.
Getting things right on paper is a good start. Making sure that the installed systems meet the requirements of the contract documents is the next step in the process. Having the commissioning provider participate in the construction phase brings the integration perspective from the drawing board to the field. The design team can focus on their individual areas of expertise while the commissioning provider targets integration, operations, and maintenance issues. It’s not unusual for the realities of the field conditions to intervene and prevent the ideal implementation of the design intent. Knowing where compromises occur allows the provider to further refine the test plan so the functional testing process can confirm that any compromise was not crippling. Participation during the construction process also allows the provider to monitor critical commissioning-related activities, such as temporary operation of equipment, factory start-ups, and verification checks. Successful execution of these areas paves the way for the functional-testing process, enabling it to become more successful by focusing on integrating and tuning the systems rather than on identifying and solving rudimentary issues like design and installation oversights.
· Functional testing ensures that the design intent is achieved by addressing integration issues from the start and providing a performance baseline useful for fine-tuning systems further.
Functional testing is where the rubber meets the road for most projects. With a well designed, well documented, properly installed system in place, the commissioning provider can focus their attention on verifying that the systems meet design intent, delivering the anticipated level of performance, functionality, and efficiency. Interaction issues and deficiencies can be identified, documented, and addressed and the systems fine-tuned. These combine to provide for a good start towards daily use of the building by the owner and its occupants.
· Trending and seasonality testing ensures the persistence of design intent. They also promote fine-tuning systems to address building-use issues over the long-term, such as seasonal integration and interaction.
Trending and seasonal testing complement initial functional testing by allowing the provider to identify and resolve the inevitable, unanticipated integration and interaction issues that become evident as the building experiences its first cycle of seasons. During this time, the systems can be fine-tuned to meet the current requirements of the owner and occupants, in addition to resolving any integration issues that are identified. All of these efforts help ensure that the design intent goals are met and will persist.
· Training and participation by the owner throughout the commissioning process further ensures the persistence of design intent and its adaptation to the changing requirements of the building owner and occupants.
Training is integral to the commissioning process and research shows that training and persistence go hand-in-hand. Owner involvement with the commissioning process provides on-the-job training regarding the integration issues and other nuances of the operation of the new building. Knowledge of the functional-testing results provides the operating team with a baseline for measuring ongoing performance. Copies of the procedures used for the initial functional test provide the operating staff with a means to test against the baseline. Participation in initial testing provides the knowledge necessary for the operating team to understand and apply testing processes, avoid the pitfalls, and interpret the results. This knowledge can be a powerful tool for ensuring the benefits of commissioning are realized and for adapting the design intent to the ever-changing needs of the building throughout its life.
The remainder of this chapter will examine how the commissioning process and functional testing can be used to identify and address integration and interaction issues and ensure that the goals established by the project’s design intent are met.