CSLB C-20 HVAC ContractorHeating SystemsMedium
A heat pump in heating mode has its outdoor coil covered in frost. The system automatically reverses to cooling mode briefly while the indoor blower continues running warm air. What component initiates this defrost cycle?
- ACompressor overload relay
- BLow-pressure switch
- CThermostat heat anticipator
- DDefrost control board sensing coil temperature and time
Show answer & explanationAnswer & explanation
Correct answer: D. Defrost control board sensing coil temperature and time
Heat pump defrost is managed by a defrost control board that monitors outdoor coil temperature (via sensor) and elapsed compressor run time, initiating a timed reversal to melt frost when both conditions are met.
Why the other options are wrong
- A. Overload relays protect the compressor from electrical overheating, not frost buildup.
- B. Low-pressure switches protect against refrigerant loss, not defrost timing.
- C. Heat anticipators are obsolete thermostat components unrelated to defrost.
Heat Pump Defrost Cycle
A timed/temperature-based control sequence that reverses the heat pump to cooling mode briefly to melt outdoor coil frost, while auxiliary heat warms supply air.
- Uses a defrost sensor (coil temp) and timer (compressor run time)
- Reversing valve energizes to send hot gas to outdoor coil
- Auxiliary/emergency heat often engages to offset cool supply air during defrost
Memory trick: Time plus temp equals thaw — the board decides the defrost law.