Heat pump not heating: guided troubleshooting
Cool or lukewarm air in heat mode. Answer each question from what you see on site.
Step 1
Is the outdoor unit running?
See every step in this flow
- Is the outdoor unit running?
- Yes: Is it in defrost right now?
- No: Outdoor unit isn't running
- The board is flashing a code: Look up the code
- Is it in defrost right now? In defrost the outdoor fan stops, the unit may steam, and the air indoors turns cool for a few minutes.
- Yes, and it ends within about 10 minutes: Normal defrost
- No: Is the outdoor coil covered in heavy ice?
- Is the outdoor coil covered in heavy ice? Light, even frost is normal. A solid block of ice is not.
- Yes: Not defrosting
- No: At the outdoor unit, is the large (vapor) line hot?
- At the outdoor unit, is the large (vapor) line hot? In heat mode the large line carries hot gas to the indoor coil.
- Yes, hot: Is the house holding temperature when it's cold outside?
- No, it's cool or cold: Reversing valve isn't in heat
- Only lukewarm: Weak heating capacity
- Is the house holding temperature when it's cold outside?
- No, it falls behind: Auxiliary heat isn't keeping up
- Yes, the air just feels cool: Probably normal
Where each path ends
Outdoor unit isn't running
Check line voltage, then for 24 V on Y at the outdoor unit. Many thermostats lock the heat pump out below a set outdoor temperature and run on auxiliary heat alone.
Look up the code
The outdoor board's status code names the circuit that tripped.
Normal defrost
A heat pump has to defrost in cold, damp weather. Auxiliary heat should temper the air during defrost; if it doesn't, check the W wiring from the defrost board.
Not defrosting
Test the defrost control and its coil sensor, the reversing valve and the outdoor fan. Low charge also causes heavy icing.
Reversing valve isn't in heat
Check whether the valve is energized as it should be for that brand (O energizes in cooling on most, B in heating on some), then whether it has shifted.
Weak heating capacity
Check airflow, then charge by the manufacturer's heating-mode method. A reversing valve leaking internally also shows up as a lukewarm vapor line.
Auxiliary heat isn't keeping up
Below the balance point the heat pump needs help. Check that the strips or furnace come on with W2, and their breakers, sequencers and limits.
Probably normal
Heat pump supply air is cooler than furnace air and can feel cool on the skin while still heating the house. Measure the temperature split to be sure.