Direct answer: If a heat pump cools fine but will not heat, the first things to check are thermostat mode, temperature setpoint, dirty filter, and whether the outdoor unit is actually running in heat mode. After that, the most likely trouble is a stuck reversing valve, a defrost problem, or backup heat that is not helping when it should.
Most likely: On most calls, this turns out to be a thermostat setting issue, airflow restriction, or a heat-pump-specific control problem rather than a total system failure.
Start by separating three lookalikes: no heat at all, some heat but not enough, and weak airflow. Reality check: heat pump supply air usually feels warm, not furnace-hot. Common wrong move: cranking the thermostat way up and assuming the outdoor unit should behave exactly like it does in summer.
Don’t start with: Do not start by replacing the thermostat, adding refrigerant, or opening electrical panels. Cooling can still work while the heating side has a different fault.
Registers are blowing steadily, but the air feels cool or barely lukewarm even after several minutes in heat mode.
Start here: Start with thermostat mode and setpoint, then confirm the outdoor unit is running when there is a call for heat.
Emergency heat works, or AUX shows on the thermostat often, but normal heat pump operation does not carry the load.
Start here: Check whether the heat pump itself is producing heat before blaming the backup heat strips.
The outdoor fan may stop and restart, frost may build up, or the unit may seem to run without delivering much heat indoors.
Start here: Look for heavy frost, ice, or a unit stuck in a bad defrost pattern.
The system runs a long time and the house temperature stalls a few degrees below the thermostat setting.
Start here: Make sure this is not really an airflow or low-capacity problem before treating it as a total no-heat failure.
A heat pump can cool normally while a thermostat is set wrong for heat mode, locked in a schedule, or not calling for auxiliary heat when needed.
Quick check: Set the thermostat to HEAT, raise the setpoint 3 to 5 degrees above room temperature, and wait several minutes to see whether both indoor airflow and outdoor unit operation match a heat call.
Heat pumps are sensitive to airflow. A loaded filter can leave you with weak heat delivery even though cooling seemed acceptable before the weather changed.
Quick check: Pull the filter and inspect it against a light. If it is packed with dust or pet hair, replace it and make sure supply and return grilles are open.
The reversing valve is what changes the refrigeration cycle between cooling and heating. If it sticks or the control side fails, the system may act like it never fully leaves cooling mode.
Quick check: With a clear call for heat, note whether the outdoor unit runs but indoor air stays cool and the larger refrigerant line never gets warm. That points away from a simple thermostat issue.
In cold weather, a heat pump needs proper defrost operation and sometimes auxiliary heat to keep up. If either side is not doing its job, the system can cool fine but struggle badly in heat mode.
Quick check: Look for heavy frost that does not clear, or a thermostat that never shows AUX during a large temperature recovery even though the house is falling behind.
You need to know the system is actually being asked to heat before you judge anything else.
Next move: If the system begins delivering clearly warmer air and the house starts climbing, the problem was likely a setting, schedule, or mode issue. If the blower runs but the air still feels cool or only slightly warm, move on to airflow and outdoor-unit checks.
What to conclude: A clean thermostat test separates a control mistake from a real heating-side fault.
Low airflow can make a heat pump feel like it is not heating even when the refrigeration side is trying to work.
Next move: If airflow improves and the supply air becomes noticeably warmer, the restriction was likely the main problem. If airflow is normal but heat is still weak or absent, the issue is more likely on the heat-pump heating side.
What to conclude: A dirty filter is common and cheap to fix. Normal airflow with poor heat points you away from the simple maintenance answer.
A heat pump that cools fine but will not heat often gives the clue outside.
Next move: If the outdoor unit runs normally, has no heavy ice, and the indoor air is warming, the system may simply be slower than expected in cold weather. If the outdoor unit does not run, ices heavily, or runs without producing warmer indoor air, you have narrowed it to a heat-pump-specific fault that usually needs service.
This tells you whether the indoor backup heat can carry the load and whether the heat pump side is the weak link.
Next move: If emergency heat warms the house but normal heat does not, the backup heat is likely okay and the heat pump heating side needs attention. If even emergency heat does not produce warmer air, the problem may include thermostat setup, indoor electric heat, or a broader control issue.
By now you should know whether this was a simple airflow issue or a true heating-side fault that is not a safe DIY repair.
A good result: If the system now heats normally after the basic corrections, keep an eye on filter condition and outdoor coil cleanliness.
If not: If the fault remains after these checks, the likely repairs are beyond basic DIY and usually involve controls, defrost components, reversing valve operation, or refrigerant-side diagnosis.
What to conclude: This is the point where you stop guessing. A heat pump that cools but will not heat usually needs targeted testing, not parts swapping.
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Because heating uses some different controls and operating conditions than cooling. A thermostat setup issue, stuck reversing valve, defrost problem, or missing auxiliary heat support can leave cooling normal while heating fails.
Yes. Wrong mode, bad programming, incorrect heat-pump setup, or a thermostat that is not calling auxiliary heat properly can all look like a heating failure. Check settings first before assuming the thermostat itself is bad.
Usually no. In normal operation it often feels warm rather than furnace-hot. If it feels room temperature or cool after several minutes in heat mode, that is when you start troubleshooting.
A light frost can be normal. Heavy ice that stays put is not. If the unit keeps icing over and does not clear itself, shut it down and get it serviced.
You can use emergency heat temporarily if the home needs heat and the system is otherwise safe, but it is usually more expensive to run. It is a workaround, not a fix for the heat pump side.
No. That repair is not basic DIY. It involves refrigerant-side work and proper system diagnosis, and it is a pro job.