WiremanWireman · Heat pump stuck in defrost or no heat: troubleshooting for electricians · 2023 NEC ·
Heat pump stuck in defrost or no heat
Reversing valve signal and coil, the defrost sensor and board, and aux heat.
Before you start
How much electrical experience do you have?
This sets how much detail each step shows. You can change it any time.
Showing detail
How it works
A heat pump heats by running its refrigerant backward through a reversing valve. A 24 V solenoid on that valve is controlled by the O or B terminal: O systems energize it in cooling, B systems energize it in heating. In winter the outdoor coil frosts, so a defrost control periodically switches to cooling, stops the outdoor fan and brings on backup heat until a sensor says the coil is clear or a time limit runs out. Refrigerant charge, valves and pressures belong to an HVAC tech.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What is the heat pump doing?
- Blows cool air in heat mode, outdoor unit runningTest: Check the reversing valve signalExpect: O systems: 0 V in heat. B systems: about 24 V in heat.
- Wrong for the modeThermostat set for the wrong reversing valve type
- Signal is correct, still blows coolTest: Test the reversing valve coilExpect: A steady reading, not 0 Ω and not OL. Magnetic pull when energized.
- OL or near 0 ΩFailed reversing valve solenoid coil
- Coil good, valve won’t shiftReversing valve stuck
- Outdoor coil is a solid block of iceDoes the unit ever go into defrost?
- Never seems to defrostTest: Test the defrost sensorExpect: A thermistor matches the chart for the coil temperature. A bimetal thermostat is closed on a frozen coil.
- Out of spec, or open on a frozen coilFailed defrost sensor
- Tests goodTest: Force a defrostExpect: The fan stops, the valve shifts and the coil starts to clear.
- Goes into defrostDefrost timing or sensor placement
- Won’t go into defrostFailed defrost control board
- Defrosts, but ice keeps buildingIcing from airflow, drainage or charge
- Never seems to defrostTest: Test the defrost sensorExpect: A thermistor matches the chart for the coil temperature. A bimetal thermostat is closed on a frozen coil.
- Steams with fan off and never comes out of itTest: Check the sensor for a stuck “cold” signalExpect: A reading that matches a warm coil. Bimetal type open.
- Reads as cold, or shortedFailed defrost sensor
- Reads correctlyFailed defrost control board
- Outdoor unit isn’t running at allThe outdoor unit isn’t running
- Cold, and emergency heat doesn’t helpBackup heat isn’t working
- Blows cool air in heat mode, outdoor unit runningTest: Check the reversing valve signalExpect: O systems: 0 V in heat. B systems: about 24 V in heat.