Heat pumps fail in cold weather, during power loss, and when refrigerant leaks or compressors break
A heat pump stops working when outdoor temperatures drop below its design limit, when ice builds up on the outdoor unit, when the compressor fails, when refrigerant leaks out, or when electrical power is lost. Some of these are temporary—defrost cycles and power restoration fix themselves. Others require a technician to replace parts or recharge the system. The most common reason a heat pump stops heating in winter is that it has entered defrost mode, which is normal and lasts 5 to 15 minutes.
Understanding which failure you are facing matters because some mean you need to call for service today, and others mean you can wait or troubleshoot yourself. A heat pump that cycles on and off normally but produces no heat in freezing weather is likely working as designed—it may have a backup heating element that should kick in automatically. A heat pump that makes noise, leaks water indoors, or stops responding to the thermostat needs a technician.
Key Takeaways
- Defrost cycles are normal and happen automatically when ice forms on the outdoor unit; the system pauses heating for 5 to 15 minutes while it melts the ice.
- Heat pumps have a minimum outdoor temperature below which they cannot extract heat from the air; most standard units stop heating effectively below 32°F without backup heat.
- A compressor failure, refrigerant leak, or electrical fault requires a technician and cannot be fixed by the homeowner.
- If your heat pump stops working during a power outage, check that power has been restored and wait 5 to 10 minutes for the system to restart; do not reset the breaker repeatedly.
- Backup heating (electric resistance or gas) should set up automatically when outdoor temperature drops; if it does not, the thermostat or control board may need service.
Defrost cycles: why your heat pump pauses in winter
When outdoor temperature and humidity create conditions for frost or ice to form on the heat pump's outdoor coil, the system automatically enters defrost mode. During this cycle, the heat pump reverses direction and uses indoor heat to melt the ice off the outdoor unit. This means you feel no heat coming from your vents for 5 to 15 minutes, and the outdoor unit may blow warm air instead of cold air.
Defrost cycles are not a malfunction—they are a built-in protection. If ice were allowed to build up, the outdoor coil would become blocked and the system would fail completely. You can tell a defrost cycle is happening by looking at the outdoor unit: you may see steam or water dripping from it, and you may hear the reversing valve click as the system changes direction. The cycle ends automatically, and heating resumes.
If defrost cycles happen very frequently (more than once per hour) or last longer than 20 minutes, the defrost control board may be stuck or the outdoor sensor may be faulty. This is worth having a technician check, but it is not an emergency.
Cold-weather cutoff: when outdoor temperature is too low
Most air-source heat pumps are designed to heat down to about 32°F outdoor temperature. Below that point, the system cannot extract enough heat from the air to warm your home efficiently. Some units are rated for lower temperatures—cold-climate heat pumps work down to 0°F or below—but standard models lose heating capacity as temperature drops.
When outdoor temperature falls below the heat pump's minimum, your thermostat should automatically switch to backup heating: either an electric resistance element (if you have a heat pump with electric backup) or a gas furnace (if you have a hybrid system). This backup heat should turn on without you doing anything. If the outdoor temperature is below freezing and your heat pump is running but your home is not warming, check your thermostat display to see whether it shows "auxiliary heat" or "emergency heat" is active. If it is not, the backup heating system may have failed.
Some heat pumps have a manual or automatic switchover point set in the thermostat. If you have a programmable or smart thermostat, you can check the settings to see what temperature triggers the switch. If the setting is wrong or the backup system is not responding, a technician will need to adjust the control board or test the backup heating element.
Refrigerant leaks and compressor failure
A heat pump that runs but produces little or no heat, or that cycles on and off very quickly, may have lost refrigerant. Refrigerant is the fluid that circulates through the system and carries heat between the indoor and outdoor units. If a line cracks, a connection loosens, or a seal fails, refrigerant escapes and the system cannot move heat. You cannot refill refrigerant yourself—it requires EPA certification and special equipment.
A compressor is the pump that moves refrigerant through the system. If the compressor fails, the heat pump will not heat or cool at all, even though the fan may still run. A failed compressor usually makes a loud grinding, squealing, or knocking noise before it stops working completely. Compressor failure is the most expensive repair in a heat pump system and sometimes means replacing the entire outdoor unit.
Both refrigerant leaks and compressor failure require a licensed HVAC technician. Do not attempt to add refrigerant or restart the system yourself. Call for service as soon as you notice the problem, because running a low-refrigerant system can damage the compressor further.
Electrical faults and power loss
If your heat pump stops responding to the thermostat, check whether power has been lost. Look at the outdoor unit to see whether the fan is running. Check your electrical panel to see whether the breaker for the heat pump has tripped. If the breaker has tripped, switch it off and then back on. Wait 5 to 10 minutes for the system to restart—heat pumps have a built-in delay to prevent damage from rapid on-off cycling.
If the breaker trips again when ready, do not keep resetting it. This usually means there is a short circuit or a fault in the compressor, and repeated resets can cause more damage. Call a technician instead.
If the breaker has not tripped but the system is not responding, check that the thermostat has power. Look for a display or indicator light. If the thermostat is dark, check its batteries (if it is battery-powered) or check the breaker for the thermostat circuit. Some thermostats have a separate breaker from the heat pump itself.
Frozen outdoor unit and ice buildup
In very humid conditions or during certain weather patterns, ice can build up on the outdoor coil faster than the defrost cycle can remove it. If you see a thick layer of ice on the outdoor unit that does not melt during a defrost cycle, the system may not be heating. This is different from a light frost, which is normal.
Do not pour hot water on the outdoor unit or try to chip away ice yourself—you can damage the coil or the refrigerant lines. Instead, turn off the heat pump and let it run in defrost mode by setting the thermostat to a higher temperature or switching it to cooling mode briefly. This forces a defrost cycle. If ice continues to build up, the defrost control board or the outdoor temperature sensor may be faulty and need replacement by a technician.
Keeping the area around the outdoor unit clear of snow and debris helps prevent ice from building up. Make sure the unit has at least 2 feet of clearance on all sides and that snow is not blocking the intake vents.
Thermostat problems and control board faults
If your heat pump is running but not responding to temperature changes, or if it is not switching between heating and cooling, the thermostat or the control board may be faulty. A thermostat that shows the wrong temperature will cause the system to run at the wrong time or not at all. A control board that has failed will not send signals to the compressor or fan.
Start by checking the thermostat display. If it is blank, check the batteries and the breaker. If the display shows a temperature that does not match the actual room temperature, the thermostat sensor may be dirty or blocked. Try moving any objects away from the thermostat and cleaning the sensor with a soft, dry cloth.
If the thermostat display is working but the system is not responding, or if you see an error code on the display, write down the code and call a technician. Different manufacturers use different codes, and a technician can look up what the code means and what repair is needed.
When to call a technician versus troubleshooting yourself
Call a technician when ready if: the compressor is making grinding or squealing noises; the system is leaking water indoors; the outdoor unit is leaking refrigerant (you will see oily residue on the coil); the breaker keeps tripping; or the system is not heating or cooling at all and the thermostat is working.
You can troubleshoot yourself if: the system is in defrost mode (wait 15 minutes); the outdoor temperature is below the heat pump's minimum and backup heat should be active (check the thermostat); the breaker has tripped once (reset it and wait 10 minutes); or the thermostat display is blank (check batteries and breaker).
If you are not sure whether the problem is normal operation or a fault, it is better to call. A technician can diagnose the issue in one visit and tell you whether repair is needed. Waiting too long on a refrigerant leak or compressor problem can cause more damage and a higher repair bill.
Frequently Asked Questions
Is it normal for a heat pump to stop heating for a few minutes?
Yes, if it is a defrost cycle. The system pauses heating for 5 to 15 minutes while it melts ice off the outdoor coil. You should feel warm air from the vents resume after the cycle ends. If the pause lasts longer than 20 minutes or happens very often, have a technician check the defrost control board.
Why does my heat pump stop working when it gets really cold?
Standard heat pumps cannot extract enough heat from air below about 32°F. Your thermostat should automatically switch to backup heating (electric or gas) at that temperature. If backup heat is not turning on, the control board or the backup heating element may need service.
What should I do if my heat pump stops working during a power outage?
Wait for power to be restored, then wait another 5 to 10 minutes before the system restarts. Do not reset the breaker repeatedly. If power is restored and the system does not restart after 10 minutes, check that the thermostat has power and that the breaker has not tripped again.
Can I add refrigerant to my heat pump myself?
No. Refrigerant requires EPA certification to handle, and adding the wrong amount or type can damage the compressor. If you suspect a refrigerant leak, call a technician. Do not run the system if you think refrigerant is low, because this can cause compressor failure.
How do I know if my heat pump compressor has failed?
A failed compressor usually makes a loud grinding, squealing, or knocking noise before it stops working. The system may run but produce no heat or cooling. If you hear these noises, turn off the system and call a technician—continuing to run a failing compressor can cause more damage.