Don’t look now, but your energy-efficiency programming isn’t saving you money anymore. The days of fixed setpoint air handlers are mostly behind us. Almost every site has, to some extent, a strategy for optimizing supply air temperature (SAT) and supply air pressure (SAP) in response to various conditions. Over the last decade Outside Air Temperature-based reset strategies have mostly been supplanted by zone-polling strategies. By far the most common is the Trim-and-Respond strategy based on VAV requests from the zones downstream of the air handler in question. Combined temperature and pressure reset strategies using zone requests tends to yield an energy-efficient and responsive building, when it’s working properly.

But it doesn’t always work properly. Improperly sized boxes, damper, valve, and sensor failures can produce requests that never go away. After a few VAVs fail, the reset strategies will drift towards their most consumptive values and stay there until an operator overrides the reset. The current approach is to adjust the number of requests that the reset logic ignores. This doesn’t target the failed VAVs, but instead makes the system less responsive overall.

The alternative to adjusting the “ignore” number is to suppress VAV requests when the VAV is not working properly. The implementation details should not be a uniform standard, because different vendors provide different details in their pre-built VAV applications. The important consideration when developing a request-suppression strategy is to ensure that false positives are very infrequent. Instead of suppressing a VAV request when a zone is 5 degrees away from setpoint (which I have seen specified), consider suppression in conditions where a damper is fully open but airflow is close to zero. Working with a controls person you trust that has experience with the controllers you plan to use is a good approach.

Even using extremely generous setpoints for fault detection, my experience over the last decade has indicated that about 40% of VAV boxes in buildings over 10 years old should not be allowed to initiate at least one type of request. Without targeted request suppression, the conventional wisdom would result in either ignoring half the plenum or manually controlling the setpoint.