Quick answer
If your heat pump stops working in cold weather, first check the thermostat settings and ensure the outdoor unit is free of ice or snow. If the issue persists, inspect the air filter and consider checking the defrost cycle. If the problem continues, a professional inspection may be needed.
Confirm the symptom
- Heat pump works in mild weather but not in cold weather
- Outdoor unit is covered in ice or snow
- Thermostat shows correct settings but no heat output
- Heat pump runs but doesn't produce enough heat
- Error codes appear on the thermostat or unit display
Tools and prep
- Thermostat
- Ladder
- Soft brush or vacuum
- Screwdriver
- Flashlight
- Gloves and safety goggles
- Manufacturer's manual
- Defrost mode activation tool (if applicable)
Step-by-step troubleshooting
- Check Thermostat SettingsDo: Ensure the thermostat is set to 'Heat' and the temperature is set higher than the indoor temperature.Observe: Thermostat displays correct settings but heat pump does not activate.Means: Thermostat may be faulty or not communicating with the heat pump.Next: Inspect the air filter and outdoor unit.
- Inspect Air FilterDo: Remove and examine the air filter for dirt or blockage.Observe: Filter is dirty or clogged.Means: Restricted airflow can reduce efficiency and performance in cold weather.Next: Clean or replace the air filter and test the heat pump again.
- Clear Outdoor UnitDo: Use a soft brush or vacuum to remove ice, snow, or debris from the outdoor unit.Observe: Unit is free of obstructions and appears to operate normally.Means: Blocked airflow can prevent the heat pump from functioning properly in cold weather.Next: Check if the heat pump starts working after clearing the unit.
- Check Defrost CycleDo: Observe the outdoor unit for signs of the defrost cycle activating (e.g., brief shutdown or reverse operation).Observe: Defrost cycle does not activate or takes too long.Means: A malfunctioning defrost system can cause the unit to ice over and stop working.Next: Consult the manual for defrost mode activation or contact a professional.
- Test Emergency HeatDo: Switch the thermostat to 'Emergency Heat' mode and see if heat is produced.Observe: Heat is produced in emergency mode but not in normal heat mode.Means: The heat pump may be unable to extract heat from the outside air due to a fault.Next: Contact a professional for further diagnosis.
- Check for Error CodesDo: Look at the thermostat or unit display for any error codes.Observe: Error codes are displayed.Means: Error codes can indicate specific issues such as refrigerant leaks or electrical problems.Next: Refer to the manual to interpret the error codes or call a technician.
Common causes
| Cause | Likelihood | Difficulty | Part often needed |
|---|---|---|---|
| Frozen Outdoor Unit Ice buildup on the outdoor coils restricts airflow and heat transfer. | Common | Easy | None (regular maintenance) |
| Dirty Air Filter A clogged filter reduces airflow, making the system work harder and less efficiently. | Common | Easy | Air filter |
| Defrost Cycle Failure A faulty defrost system can't remove ice from the outdoor unit, causing it to stop working in cold weather. | Sometimes | Medium | Defrost timer or control board |
| Thermostat Issues A malfunctioning thermostat may not signal the heat pump to activate properly. | Sometimes | Easy–Medium | Thermostat |
| Low Refrigerant Low refrigerant levels reduce the system's ability to transfer heat, especially in cold conditions. | Sometimes | Hard | Refrigerant and repair service |
| Electrical Problems Faulty wiring or a tripped circuit breaker can prevent the heat pump from operating. | Rare | Medium | Electrical components or circuit breaker |
| Compressor Failure A failed compressor cannot circulate refrigerant, leading to no heat production. | Rare | Hard | Compressor replacement |
Parts matching
If you need to replace parts, here are common items for this issue:
- Air filter (replace every 1–3 months)
- Thermostat (check for proper calibration)
- Defrost timer or control board (if defrost cycle is not working)
- Refrigerant (if low refrigerant is diagnosed)
- Compressor (if compressor failure is confirmed)
Always refer to your heat pump's manual for specific part compatibility and installation instructions.
When to call a professional
- The heat pump still doesn't work after clearing the outdoor unit and replacing the air filter
- Error codes persist or indicate a refrigerant leak
- Defrost cycle does not activate or takes too long
- Emergency heat works but normal heat does not
- You suspect electrical issues or compressor failure
- You are unsure about any step or feel unsafe performing repairs
FAQ
Why does my heat pump stop working when it gets cold?
Heat pumps struggle in cold weather because they extract heat from the air. If the outdoor unit freezes or the system is inefficient, it may not produce enough heat. Check for ice buildup, dirty filters, or defrost cycle issues.
Can I use my heat pump below freezing temperatures?
Most heat pumps are designed to work efficiently down to about 25–30°F. Below that, they may rely on auxiliary or emergency heat. If it's much colder, the system may not function well without supplemental heating.
How do I know if my heat pump's defrost cycle is working?
During cold weather, the defrost cycle should activate periodically. You'll notice the outdoor unit briefly stop or run in reverse to melt ice. If this doesn't happen, the defrost system may be faulty.
Should I turn off my heat pump in very cold weather?
No, keep the heat pump on. Turning it off can cause more ice buildup. Instead, ensure the outdoor unit is clear of snow and debris, and check the defrost cycle.
What is emergency heat, and when should I use it?
Emergency heat is a backup heating source (usually electric resistance coils) that kicks in when the heat pump can't provide enough heat. Use it only when the heat pump is not working, and turn it off once the issue is resolved to save energy.