A heat pump in cooling mode works like an air conditioner, so most cooling problems are the same. There is one extra part that can fail: the reversing valve that switches the system between heating and cooling. Here is how to work through it.
Check the thermostat first
- Mode set to COOL, not HEAT, AUTO or EM HEAT.
- Setpoint below room temperature.
- Fan set to AUTO. On ON, the blower runs between cycles and moves room-temperature air that feels warm.
- Wait for the delay. A blinking Cool On is usually a normal compressor delay of up to five minutes. See thermostat blinking Cool On.
Check the basics
- The filter. Replace it if you cannot see light through it.
- The outdoor unit. Is it running? Is the fan spinning? Is the coil clean and clear of debris?
- The breakers and disconnect. The outdoor unit has its own breaker and a disconnect box on the wall.
- The vents. Open all supply registers and clear the return grille.
- Ice. Frost on the refrigerant line or the indoor coil means turn cooling off and set the fan to ON until it thaws.
The usual causes
Stuck reversing valve
The part unique to heat pumps. If it sticks in the heating position, the system runs and blows warm air even with the thermostat on cool. Sometimes it is the valve itself, sometimes its solenoid coil or the signal from the thermostat. If your system blows warm air in cooling and cool air in heating, the valve or its control is the likely cause.
Low refrigerant
The system runs but the air is only slightly cool, the coil may ice, and the house never reaches setpoint. Refrigerant is not used up, so low charge means a leak. See refrigerant costs and the R-410A phase-out.
Failed capacitor or outdoor fan motor
The indoor fan runs but the outdoor unit hums or sits still. The indoor air feels warm because the compressor is not running.
Dirty outdoor coil
Common in our valley by midsummer. The unit cannot shed heat, capacity falls and it may shut off on high pressure.
Restricted airflow indoors
A dirty filter, a dirty indoor coil or a failing blower cuts capacity and can freeze the coil.
Leaking attic ducts
The equipment works, but cold air escapes into the attic before reaching the rooms. See an AC that runs but will not cool the house.
A quick test you can do
Put a thermometer in the return grille and another in a supply vent near the air handler. After fifteen minutes of cooling, a healthy system shows roughly 16 to 22 degrees of difference. Much less than that points to an equipment problem. A normal split with a hot house points to the ducts or the house itself.
When to stop and call
- The outdoor unit hums without the fan turning
- A breaker trips a second time. See tripping the breaker.
- Ice returns after thawing
- Warm air in cooling mode with everything else normal
What we do
We check the reversing valve and its control, verify the refrigerant charge, test the capacitor and fan motor, and measure airflow and temperature split. Our diagnostic is a flat $69 and it is credited toward the repair. Related: AC blowing warm air, heat pump freezing up, and our heat pump services page.
Frequently asked questions
Why is my heat pump not cooling?
Common causes are a thermostat setting, a clogged filter, a dirty outdoor coil, a failed capacitor or fan motor, low refrigerant from a leak, or a reversing valve stuck in heating.
Why is my heat pump blowing warm air in cooling mode?
Often a reversing valve stuck in the heating position or a problem with its control. It can also mean the compressor is not running, so the fan is just moving room air.
What is a reversing valve?
The valve that changes the direction of refrigerant flow so a heat pump can heat or cool. If it sticks, the system may heat when you want cooling.
Can a heat pump lose refrigerant?
Only through a leak. Refrigerant is not consumed, so a low charge means there is a leak to find and repair.
How cold should air from a heat pump be in cooling?
Roughly 16 to 22 degrees cooler than the air entering the return, after the system has run for fifteen minutes.
How much does it cost to fix a heat pump that is not cooling?
Our diagnostic is $69 and credited toward the repair. A capacitor or fan motor is a moderate repair. A reversing valve or refrigerant leak costs more, and we quote before any work.