Tomorrow's Engineer: Why Seasonal System Performance Testing?
If the commissioning engineer is brought on during the construction phase, it is very difficult to convince a contractor to defer the demonstration of one of these systems until the peak heating or air conditioning season. The contractor is anxious to close out the job on time and in budget. They do not want to have the owner acceptance be delayed four or six months while everyone waits for the correct outdoor air temperatures to arrive. The reasons can be many, but withholding final payment, owner sign-off for the project, and/or determining and designating the starting time for equipment warranty, are right up there at the top of the lists.
If the commissioning engineer is hired during the design phase, then the contract bid documents can be written to mandate deferred/seasonal performance testing, obligating the contractor to demonstrate the system in the correct season. Within this specification can be the deferred seasonal functional performance requirements that mandate the specific system be demonstrated to the owner in the appropriate season even if the rest of the building has been completed and the owner has taken over occupancy of this facility.
Here are some reasons why a building owner should want to start the commissioning process in the design phase and get the seasonal functional performance test requirement in the contract:
- Stratification can't be simulated. When we have a contractor demonstrate a central air-handling system during the air conditioning season, we can change control setpoints to trick the heating coil control valve to sequence through its modulation range from closed to 100% open and then reverse the process. This test is fine to see if the programming is in place, but you can't simulate stratification of cold outdoor air mixing (or not mixing) with warm return air. So how important is this? Stratification is not only a nuisance, it can be costly if a coil freezes because the freeze-stat didn't pick up this winter problem. Stratification can only be observed during the colder weather.
- Verifying equipment capacity in the heating season. While observing the engineered smoke controlled system on-site with the outdoor air temperature at -15°F, we watched the supply discharge air temperature progressively drop off from its 55° setpoint, eventually shutting down when the freeze-stat signaled low temperature. Upon inspection of the integral face and bypass preheat coil, we found the coil size was not correct and did not meet the shop drawing submittal. It wasn't until the system was run through its paces that cold winter day that the obvious became obvious: the preheat coil couldn't meet the heating season criteria.
- Carryover moisture from the cooling coil. Here is another seasonal phenomenon: moisture carryover beyond the cooling coil drain pan. This was an observation made during the peak air conditioning season that couldn't be observed if the central AHU functional performance test was completed during the heating season. According to the equipment manufacturer, this should not have occurred because its equipment was factory tested. However, when you are standing next to the AHU with water dripping out of the fan casing, it is tough to accept "factory tested." You can exercise the chilled water valve performance year-round, but without humid air entering the coil and maximum supply air from the air handler, moisture carryover would have been missed.
- Verifying equipment capacity in air conditioning season. Just like the example above where we observed the integral face and bypass preheat coil capacity performance, the only way to verify cooling coil performance is during the air conditioning peak season. You can't simulate peak cooling when the entering air is a dry mixture of outdoor air and return air during the heating season. What a system functional performance demonstration needs when verifying the cooling coil capacity is hot, humid entering air.