Why Is My Heat Pump Not Heating? (7 Causes & Fixes)
Heat pump on but not heating this winter? Start with thermostat mode and a $5 filter before calling anyone. Here are 7 causes and what each one looks like.
Quick answer: Most heat pumps that stop heating in fall or winter have a thermostat still set to “cool,” a dirty filter choking airflow, or an outdoor unit running through a defrost cycle. Check those three before anything else. If the system powers on normally but blows room-temperature air even after the outdoor unit thaws, you’re likely looking at low refrigerant or a failed reversing valve, both of which need a licensed technician.
Heat pumps work differently from furnaces. Instead of burning fuel, they pull heat from outdoor air and move it inside, which works well down to around 25-30°F for most standard models. Below that, efficiency drops and backup electric heat strips handle the gap. If yours isn’t heating at all, the fix is usually something simple that takes five minutes to check.
7 Reasons Your Heat Pump Is Not Heating
1. Thermostat Still Set to “Cool”
The most common November service call. The thermostat spent summer in cooling mode and nobody switched it. Check the mode selector on the thermostat face. It should read “heat,” not “cool.”
Also check for “EM HEAT” or “emergency heat” lit on the display. That setting bypasses the heat pump entirely and runs only the backup electric resistance strips. Those strips cost 2-3 times more per hour to run than the heat pump itself. Emergency heat is meant for when the heat pump has physically failed, not for cold days. If yours is on emergency heat unnecessarily, flipping it back to regular “heat” mode will restore normal operation and cut your electricity cost.
If you want something that makes this mistake harder to repeat, the best smart thermostats for older homes can alert you remotely when the mode is wrong.
2. Dirty Air Filter
A clogged filter restricts airflow through the air handler, cutting the system’s ability to exchange heat. Some filters get packed so tight they’re barely letting air through at all. The blower runs, the refrigerant moves, but there’s nowhere for the conditioned air to go.
Carrier and Trane both recommend checking filters monthly during peak heating and cooling seasons. A standard 1-inch filter needs replacing every 30-90 days depending on pets, allergies, and dust levels. A 4-inch media filter can go 6-12 months. Pull yours and hold it up to light. If you can’t see through it, that’s a $5-15 fix that often restores full heating capacity.
For the full schedule and right MERV rating for different situations, see how to change your HVAC filter.
3. Frozen Outdoor Unit
Some frost on the outdoor coils is normal in temperatures between 25-45°F with high humidity. The system handles this automatically with periodic defrost cycles that reverse refrigerant flow briefly and melt the ice. You’ll see the outdoor fan stop, possibly steam rising from the unit, then it resumes normal operation. That’s the system working as designed.
The problem is a unit completely encased in ice that doesn’t clear after a defrost cycle. That usually points to one of these:
- Restricted airflow around the unit (leaves, debris, or fencing closer than 18 inches on the sides)
- Running continuously in sustained temperatures below 20°F without adequate defrost
- Low refrigerant charge (see cause 5 below)
If the outdoor unit is visibly frozen solid, turn the system off at the thermostat and let it thaw naturally. Don’t chip at the coils with a screwdriver; the aluminum fins are thin and damage easily. Once it’s thawed, run it for a day and see if freezing resumes. If it does, call a technician.
4. Tripped Circuit Breaker
Heat pump systems typically run on two separate breakers: one for the indoor air handler and one for the outdoor compressor. If the outdoor unit’s breaker trips, the air handler might still run and move air, but nothing is heating it.
Check the main panel. If a breaker is tripped (pointing sideways or showing a red indicator), flip it fully to OFF first, then back to ON. Reset it once. If it trips again within an hour, stop. A breaker that keeps tripping is signaling an underlying electrical problem, often a failing compressor capacitor or a wiring fault. Repeated resets can damage the compressor. That’s a job for a technician.
5. Low Refrigerant (Refrigerant Leak)
A heat pump low on refrigerant will blow air that’s cool or only mildly warm even when running continuously. The refrigerant is what carries heat between the indoor and outdoor coils. A moderate leak can cut heating capacity by half.
Signs pointing toward low refrigerant:
- System runs constantly but can’t reach set temperature
- Ice forming on the outdoor unit even when it’s not that cold outside
- Hissing or bubbling sounds near the refrigerant lines or outdoor unit
Most heat pumps use R-410A. Newer systems use R-32. Under EPA Section 608 of the Clean Air Act, handling either refrigerant requires technician certification. You can’t add it yourself, and shouldn’t want to: adding charge without finding the leak just postpones the same problem. A licensed HVAC tech needs to find the leak, repair it, pressure test, and recharge the system.
6. Faulty Reversing Valve
The reversing valve is what makes a heat pump different from a standard air conditioner. It redirects refrigerant flow to switch between heating and cooling modes. When it fails, the system usually gets stuck in one mode, often cooling, which means blowing cold air even with the thermostat set to heat.
A reversing valve failure is usually recognizable because the system worked fine in one season and fails in the other. Ran great all summer, won’t heat now. The valve is brazed directly into the refrigerant circuit, and replacing it requires EPA-certified technician work. This isn’t a DIY fix.
For more context on how heat pump and AC diagnostics overlap, the AC not blowing cold air article covers the summer-side version of similar problems.
7. Outdoor Temperature Below Efficient Range
Standard heat pumps work well down to around 25-30°F outdoor temperature, then efficiency drops off and backup heat carries more of the load. Cold-climate heat pumps from Mitsubishi (Hyper Heat line), Bosch, and Daikin are rated to operate efficiently down to -13°F, but most builder-grade units from the 1990s and 2000s were standard-efficiency models not designed for severe winters.
If your house is cold and the heat pump runs continuously but can’t keep up, check the outdoor temperature. If it’s below 20°F and you have an older standard-efficiency unit, the system may be doing everything right and simply needs backup heat to close the gap. Most dual-fuel systems (heat pump plus gas furnace) automatically hand off to gas at a preset balance point, usually around 35-40°F.
If the backup heat strips aren’t engaging on cold nights, the problem might be in the thermostat settings or a secondary control board defect rather than the heat pump itself.
Common Mistakes
Turning it off at the breaker then resetting it again. One reset is reasonable. If the breaker trips again within an hour, stop. Forcing a struggling compressor to restart against an unresolved electrical fault shortens its life. The breaker is doing its job.
Using emergency heat because it’s cold outside. Emergency heat bypasses the heat pump and runs only the backup strips. It costs 2-3 times more per hour than the heat pump. Emergency heat is for genuine breakdowns, not cold mornings.
Closing vents in unused rooms. Heat pump manufacturers recommend keeping at least 80% of supply vents open. Closing vents builds pressure in the duct system, reduces airflow past the coils, and can cause the outdoor unit to ice up faster.
Ignoring the defrost cycle. The unit cycling off, the fan stopping, steam rising for a few minutes every 90 minutes is normal defrost operation. If you shut the system off every time this happens, the coils stay frozen and the system can’t recover.
Assuming low refrigerant is routine maintenance. Unlike engine oil, refrigerant doesn’t get used up. If your system is low on charge, there’s a leak somewhere. Recharging without fixing the source is temporary.
When to Call a Pro
Call a licensed HVAC technician when:
- The outdoor unit is completely frozen and refreezes within 24 hours of thawing
- The breaker trips again after one reset
- You hear hissing, bubbling, or grinding from the system
- The system runs continuously but can’t reach temperature, even after checking the thermostat, filter, and breaker
- The unit is 10+ years old and showing declining performance; at that age, a failing compressor or reversing valve often makes replacement more cost-effective than repair
Refrigerant work and reversing valve replacement require EPA certification. These aren’t DIY jobs regardless of your comfort level with HVAC systems.
For broader HVAC diagnostic coverage, the HVAC troubleshooting guide walks through furnace, air conditioning, and duct problems with estimated DIY versus tech-call costs.
FAQ
What does it mean when my heat pump is running but not heating? The system is operating but not moving enough heat. Start with the thermostat mode (should be on “heat,” not “cool” or “emergency heat”), then check the air filter. If those are fine and the outdoor unit looks normal, low refrigerant or a stuck reversing valve are the next possibilities and both need a technician.
Why is my heat pump blowing cold air when the heat is on? If the air feels room temperature or colder with the thermostat set to heat, the reversing valve is the most common culprit. It’s the component that switches the system between heating and cooling modes. When it fails, the system often locks in cooling mode. A technician can test it in about 20 minutes.
How cold is too cold for a heat pump? Standard models lose efficiency below 25-30°F outdoor temperature. Cold-climate models (Mitsubishi Hyper Heat, Bosch, some Daikin units) work efficiently down to -13°F. If your system struggles on the coldest days, it may be sized for milder winters than yours. A dual-fuel setup with gas backup handles extreme cold days while the heat pump covers everything else.
Can I add refrigerant to my heat pump myself? No. EPA Section 608 requires technician certification to handle refrigerants. Beyond the legal requirement, adding refrigerant without finding the source of the leak is a temporary fix. The charge will drop again. A licensed tech needs to locate the leak, repair it, and recharge the system properly.
Why does my heat pump freeze up in winter? Some frost on the outdoor coils is normal in cold, humid conditions and the system defrosts automatically. A unit encased in ice that won’t clear after defrost cycles point to low refrigerant, a defrost control board failure, or restricted airflow around the outdoor unit. All three warrant a technician visit.
Is emergency heat the same as heating mode? No. Emergency heat bypasses the heat pump and runs only the backup electric resistance strips, which cost roughly 2-3 times more per hour to operate than the heat pump. Emergency heat is for when the heat pump has mechanically failed. Running it as a regular heat source will increase your electric bill noticeably.
My heat pump cooled fine all summer but won’t heat now. What failed? That’s a classic reversing valve pattern. The valve physically switches the direction refrigerant flows to put the system in heating or cooling mode. When it sticks or fails electrically, the system locks in one mode. A technician can diagnose it with straightforward tests.
Conclusion
For most homeowners, a heat pump that stops heating in fall is a thermostat still on “cool” or a filter that hasn’t been changed since summer. Those take five minutes and $5-15 to fix. A frozen outdoor unit usually resolves itself through the defrost cycle once airflow around the unit is clear.
The harder problems, low refrigerant, a failed reversing valve, an aging compressor, require a technician. If the system is more than 15 years old and has needed refrigerant work before, the U.S. Department of Energy notes that heat pumps with older technology cost significantly more to operate than current high-efficiency models. At that point, replacement often makes more sense than ongoing repairs.
If you’re also troubleshooting a gas furnace, why is my furnace blowing cold air covers the same diagnostic logic for the gas side.