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.

Precise sequencing prevents energy waste by ensuring the preheat control thermostat is set several degrees below the design cooling coil off-coil temperature. This deadband guarantees that heating steam and cooling chilled water valves never actuate simultaneously within the single zone air handler.