The fan coasting feature (FanSuCstVal) prevents premature closing of the room supply damper while the AHU fan is coasting down. This might occur during a switch between operating modes (for example, switching from Comfort to Economy).
The fan coasting feature is active when:
- The room supply air damper is the only damper that is open – all other VAV supply air dampers are closed.
- The AHU supply fan has been turned off and is coasting down.
- The airflow setpoint signal (VavSuSpAirFl), while on its way to zero, falls below the airflow demand hysteresis setting (HysAirFlDmd).
When these conditions are true, the boolean supply fan coasting signal (FanSuCstVal) will equal 1 (True) and the damper is reset to its last (previous) operating position / value. This keeps the damper open while the fan coasts down to prevent supply duct over-pressure.
In addition to the fan coasting signal freezing the damper in place, an internal timer function was activated when VavSuSpAirFl initially dropped below HysAFlSta. The fan coasting signal and the timer (60 seconds) must both finish (return to False) before the room supply damper can close. See the figure below.
The internal timer operates independently and regardless of FanSuCstVal. Once the timer is activated, the room supply air damper is frozen in place until the counter reaches zero, after which the damper may close.
Fan supply (FanSuCstVal) coast value comes from the primary AHU.