As promised in February's column, this month I'm going to share some of the most common deviations between specified sequences of operation and implemented sequences of operation discovered during air-handling system functional performance testing. These are "surprises" in the fact that the programmed sequence of operation is not documented as being different from or in addition to the design requirements in the control system submittal.

Mixed Air, Low-Temperature Limits

Outside air and return air dampers have historically been modulated in opposition to each other during economizer operation to provide a setpoint mixed air temperature. With the advent of prescriptive minimum outside airflow quantities for IAQ, the control of mixing dampers has moved away from temperature control as the primary driver to outside air quantity as the primary control parameter. This means that the outside air and return air dampers are often specified to be modulated to maintain a setpoint outside air intake rate (cfm).

We have seen this often in recent years, but many controls contractors are programming overrides to the outside airflow control to prevent the mixed air temperature from falling below a "dangerous" level. In many VAV systems serving occupied spaces, it is not unlikely that the setpoint outside airflow rate will result in very low mixed air temperatures during extremely cold weather. This should be factored into the engineer's design with appropriate freeze protection components to "handle" the low air temperatures entering the air handler. In these cases, preventing the mixed air temperature from falling "too low" will result in lower-than-designed ventilation rates. Although the programmer includes this "enhancement," because they believe it is best for the owner, that decision is not the contractor's to make.

Lead/Lag Pumps

We almost always see hydronic pump systems specified with lead/lag redundancy and the requirement that the lag pump automatically start whenever the lead pump is sensed to be off. However, we almost never see a specified sequence of operation which defines what happens when the lead pump is restored to operation (after being repaired, reset, etc.). That means, of course, that we almost never see a control system submittal that defines that "return to normal" sequence either.

Usually, one of two scenarios is programmed by the controls contractor, based on his training and experience: (1) The lead pump resumes operation as the lead pump and the lag pump stops; (2) The original lag pump is automatically changed to be the new lead pump and the original lead pump reverts to the lag position; thus, the original lead pump is off upon return to normal operation and the original lag pump remains on.

Unlike the previous example, in most cases it really doesn't matter what happens when the lead pump is restored. The key point for the commissioning consultant is to document the control strategy for the future operators, because they will need to understand what to expect in that situation.

Outside Air Temperature Lockouts

A carryover from the old pneumatic/electric days is the concept that it is necessary to have two modes of operation - summer and winter - to help ensure that undesired simultaneous heating and cooling does not occur between two control loops that don't "talk" to each other. Thus, there was a normal strategy of defining an outside air temperature above which the heating controls would be disabled and another outside air temperature below which the cooling controls would be disabled.

That approach isn't needed with EMS, because every device on the system can "know" what every other device is doing. Nonetheless, we find many instances where the controls contractor has programmed some device lockouts based on outside air temperature, even when those lockouts were not specified by the designers. The modification is not necessarily a problem, but, again, it is critical that the operators know that this is buried in the software if they are going to understand why things are happening (or not happening) when they take over the building.

A classic example of when this can be problematic, though, is the case of a hydronic heating system which is intended to operate year-round for dehumidification reheat. If the programmer includes a lockout of the hot water pumps above 50 degrees F (we find this frequently), the building is going to be too cold and too humid - exactly the opposite of the design intent. ES