In the illustrated single zone HVAC system, what prevents the simultaneous flow of steam through the preheat coil and flow of chilled water through the cooling coil? See Illustration GS-RA-09 Illustration GS-RA-09: the U.S. Coast Guard has not released this illustration.

Correct Answer: Option A — The supply air duct thermostat controlling the steam flow is set several degrees lower than the design off-coil temperature associated with the cooling coil.

To prevent energy waste and simultaneous heating and cooling, the preheat coil thermostat is set lower than the cooling coil's off-coil temperature. This ensures the preheater only activates when the air temperature falls below the required minimum, preventing the cooling coil from fighting the heat added by the preheat stage.