Common reasons your heat pump stops producing heat
A heat pump that runs but doesn't heat usually has one of five problems: the thermostat is set to the wrong mode, the outdoor unit is frozen or blocked, the refrigerant is low, the reversing valve is stuck, or an indoor component has failed. Most of these you can check yourself in under ten minutes. Start by looking at your thermostat display—if it says "Cool" or "Fan Only" instead of "Heat," that's your answer.
If the thermostat is already set to Heat mode and the system is still blowing cold air, move to the outdoor unit. Walk outside and look at the fan and coils. Ice buildup on the outdoor unit is normal during defrost cycles (the system melts frost every 30 to 90 minutes), but thick ice that doesn't melt away within an hour suggests a problem. Leaves, dirt, or snow blocking the unit will also stop heat output.
Key Takeaways
- Check your thermostat first—it should be set to Heat mode, not Cool or Fan Only, and the temperature setting should be higher than the current room temperature.
- Look at your outdoor unit for ice buildup, blocked vents, or debris; clear away leaves and snow, and let the defrost cycle run if you see light frost.
- If the indoor fan runs but no warm air comes out, the reversing valve or indoor coil may have failed and will need a technician.
- Heat pumps lose heating power below 32°F and may need a backup heat source; check whether your system has electric resistance heating available.
- Low refrigerant causes weak heating and requires a licensed technician to locate the leak and refill the system.
Thermostat settings that stop heat output
Your thermostat has several modes, and the wrong one will make you think the heat pump is broken. Look at the display and find the mode selector—it usually shows Heat, Cool, Auto, or Fan Only. If it's set to Cool, the system will blow cold air no matter how cold your house gets. If it's set to Fan Only, the fan runs but the compressor (the part that creates heat) stays off.
Even if the mode says Heat, check the temperature setting. The thermostat only triggers heating when the room temperature drops below the set point. If you set it to 68°F but the house is already 70°F, nothing will happen. Raise the setting to 72°F or higher and wait five minutes. You should hear the compressor start and feel warm air from the vents within a minute or two.
Some thermostats have a separate switch for Emergency Heat or Auxiliary Heat. This is electric resistance heating (like a space heater built into your system) and uses much more electricity than the heat pump itself. If that switch is on, turn it off unless outdoor temperatures are below 32°F or the heat pump has failed completely.
Frozen outdoor units and blocked vents
Heat pumps pull heat from outside air even when it's cold. To do this, they run refrigerant through the outdoor coils, which makes those coils colder than the air around them. Moisture in the air freezes on the coils. The system has a defrost cycle that reverses the flow every 30 to 90 minutes to melt the ice, but heavy frost or ice buildup can block airflow and reduce heating.
Walk outside and look at the outdoor unit. If you see a thin layer of frost, that's normal—the defrost cycle will handle it. If ice is thick or hasn't melted in over an hour, turn off the system at the thermostat and let it sit for 30 minutes. This stops the compressor and lets any ice melt naturally. Then turn it back on. If ice returns within an hour, the defrost sensor or reversing valve may be faulty and you'll need a technician.
Check for blocked vents around the unit. Leaves, grass clippings, snow, or debris covering the intake or exhaust will choke airflow. Clear away anything within 12 inches of the unit. If snow has piled up around it, push the snow back. Make sure the unit sits on level ground—if it's tilted, water can pool and freeze inside.
Low refrigerant and leaks
Refrigerant is the fluid that moves heat from outside to inside. If the system is low on refrigerant, it can't move enough heat, and you'll feel weak or cold air even when the compressor is running. Low refrigerant usually means there's a leak somewhere in the copper lines, the coils, or the connections.
You can't refill refrigerant yourself—it requires a licensed technician with special equipment. But you can spot the signs: the indoor coil (usually in a closet or attic) will frost over, or you'll see oil stains on the copper lines running between the indoor and outdoor units. If you see either, call a technician. They'll find the leak, repair it, and refill the refrigerant. This typically costs between $300 and $1,000 depending on the leak location and the type of refrigerant your system uses.
Reversing valve failure
A reversing valve is a small part inside the outdoor unit that switches the direction of refrigerant flow. In heating mode, it sends hot refrigerant indoors. If this valve gets stuck or fails, the system may run but blow cold air because the refrigerant is flowing the wrong direction.
You can't fix a reversing valve yourself. The sign that it's failed is usually that the system runs, the outdoor unit fan spins, but the indoor air stays cold or barely warm. Some technicians can unstick a stuck valve by tapping the outdoor unit gently or running a defrost cycle, but if it's truly broken, the valve needs replacement. This costs $400 to $800 in most cases.
Heat pump performance below freezing
Heat pumps become less efficient as outdoor temperature drops. Below 32°F, a heat pump pulls in less heat from the outside air, and you may notice the system takes longer to warm the house or doesn't reach your set temperature. This is normal operation, not a failure.
Most heat pump systems include electric resistance heating (also called backup heat or auxiliary heat) for these cold periods. This is a heating element similar to a space heater that turns on automatically when outdoor temperature drops below a set point (usually 32°F to 35°F). It uses more electricity than the heat pump, so your bill will rise on very cold days, but it ensures your house stays warm.
If your house isn't heating at all during cold weather, check whether your system has backup heat and whether it's enabled. Look at your thermostat or system manual. If backup heat is available but not turning on, the sensor that triggers it may have failed—this requires a technician visit.
When to call a technician
Call a licensed HVAC technician if you've checked the thermostat, cleared the outdoor unit, and the system still isn't heating. Also call when ready if you see ice that won't melt, oil stains on the refrigerant lines, or if the system runs but produces no warm air at all.
Before the technician arrives, write down what you've already checked: the thermostat mode and temperature setting, whether the outdoor unit has ice or debris, and how long the system has been running without heat. This information helps the technician narrow down the problem and may save you a service call fee if the fix is straightforward.
Frequently Asked Questions
Why is my heat pump blowing cold air in heating mode?
The most common cause is a stuck reversing valve, which sends refrigerant the wrong direction. Other causes include low refrigerant from a leak, a frozen outdoor unit that hasn't completed its defrost cycle, or a failed indoor blower motor. Check the outdoor unit for ice first; if it's clear and the thermostat is set correctly, you'll need a technician to diagnose the valve or refrigerant level.
Should I see frost on my outdoor heat pump unit?
Light frost is normal and expected. The defrost cycle melts it every 30 to 90 minutes. Thick ice that builds up quickly or doesn't melt within an hour suggests the defrost sensor or reversing valve is faulty. If you see only light frost and the system is heating normally, nothing is wrong.
Can I fix a heat pump that won't heat myself?
You can clear debris from the outdoor unit, check the thermostat settings, and let ice melt naturally. You cannot refill refrigerant, repair a reversing valve, or replace internal components—these require a licensed technician and special tools. Attempting repairs yourself may damage the system or void the warranty.
Why does my heat pump use so much electricity in winter?
When outdoor temperature drops below 32°F, the backup electric resistance heating turns on to supplement the heat pump. This heating element uses much more electricity than the heat pump alone. This is normal operation on cold days, though it will raise your heating bill compared to fall and spring.
How long does it take a heat pump to warm a cold house?
A heat pump typically raises room temperature by 1°F to 2°F per hour when running continuously. If your house is 55°F and you set the thermostat to 70°F, expect 8 to 15 hours of heating. On very cold days, backup heat will turn on to speed this up, but it uses more electricity. If heating is much slower than this, the system may have low refrigerant or a failing component.