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.
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.