Heat Pump Not Cooling? Causes and How to Fix It

Last updated: July 2026 · Reviewed against Carrier, Goodman and Mitsubishi service documentation

A heat pump not cooling usually points to a thermostat setting, a dirty filter or coil, low refrigerant, or a stuck reversing valve. Since a heat pump is essentially an air conditioner that can run in reverse, most cooling problems mirror AC problems. Here is how to find the cause, from the quick DIY checks to the parts that need a technician.

Thermostat and Settings First

Set the thermostat to COOL and the target several degrees below room temperature. Confirm the fan is on AUTO, not ON (on ON the blower runs even when the compressor is off, blowing room-temperature air that feels like it is “not cooling”). Replace the batteries if the display is dim.

Dirty Filter and Coils

The most common DIY-fixable cause. A clogged air filter chokes airflow and cripples cooling. Beyond the filter, both the indoor evaporator coil and the outdoor condenser coil need to be reasonably clean to transfer heat. Clear leaves, grass, and debris from around the outdoor unit, and replace the filter. A badly dirty coil needs professional cleaning.

Frozen Indoor Coil

Ironically, a heat pump that is not cooling may have a frozen evaporator coil — ice blocking airflow. It is usually caused by low airflow (dirty filter) or low refrigerant. Turn the system to fan-only for a few hours to thaw it, replace the filter, and see if cooling returns. If it freezes again, you likely have a refrigerant problem.

Low Refrigerant (Pro Territory)

If airflow is fine but the air is not cold, low refrigerant from a leak is a prime suspect. Heat pumps do not consume refrigerant, so a low charge always means a leak that must be found and repaired — topping it off without fixing the leak is a temporary and wasteful fix. This needs a licensed technician.

Stuck Reversing Valve

The reversing valve switches the heat pump between heating and cooling. If it sticks, the system may blow warm air in cooling mode (or cool air in heating mode). A stuck valve needs professional diagnosis. If your heat pump also struggles in winter, a faulty reversing valve could be behind both — see heat pump not heating.

When to Call a Pro

Call an HVAC technician if the coil keeps freezing, you suspect low refrigerant, the outdoor unit will not run, or the reversing valve is stuck. Clean filters and coils first — many “broken” heat pumps just need airflow restored. Diagnostic visits run $90–$180.

FAQ

Why is my heat pump running but not cooling?

The unit runs but does not cool most often because of a dirty filter restricting airflow, a frozen evaporator coil, or low refrigerant from a leak. Check and replace the filter first, then look for ice on the indoor coil.

Why is my heat pump blowing warm air in cooling mode?

Warm air in cool mode points to low refrigerant, a stuck reversing valve, or the outdoor unit not running. First confirm the thermostat is on COOL and the fan on AUTO, then check that the outdoor compressor is actually running.

Should I turn off a heat pump that is not cooling?

If the indoor coil is frozen, yes — switch to fan-only to thaw it and prevent compressor damage. Otherwise, running a heat pump that cannot cool wastes energy; shut it off until you have checked the filter or called for service.