Most common causes
- Defrost mode. On cold, damp nights the outdoor coil frosts over. The heat pump switches to cooling for a few minutes to melt it, and you may feel cool air and see steam outside. That is normal.
- Normal heat pump air temperature. Heat pump air feels warm, not hot like a gas furnace. Air around body temperature from the vents can still be heating the house.
- Thermostat set wrong. A thermostat set to Cool, Auto, fan On, or one not set up for a heat pump will not control the heat properly.
- Reversing valve or its coil failed. The reversing valve switches the system between heating and cooling. If it sticks, the system can cool when it should heat.
- Defrost control problem. If the defrost board or sensor fails, the outdoor coil stays covered in ice and heating drops off.
- Refrigerant leak. A low charge reduces heating just like it reduces cooling. The leak must be found and fixed.
- Backup heat not working. Many heat pumps use electric heat strips on very cold nights. If those or their breaker fail, heat drops when it is coldest.
What you can safely check
- Set the thermostat to Heat, raise the setpoint a few degrees, and set the fan to Auto.
- If you see steam from the outdoor unit and the cool air stops within about 10 to 15 minutes, it was a defrost cycle.
- Replace the air filter if dirty.
- Check the breakers for both the outdoor unit and the indoor air handler, which often has a separate breaker for heat strips.
- Clear debris and snow or leaves from around the outdoor unit. Do not chip ice off the coil.
Call a technician if
- Cold air continues for more than 20 minutes in heat mode
- The outdoor unit is fully covered in ice and stays that way
- The system heats in cooling mode or cools in heating mode
- The emergency or auxiliary heat light stays on
- Breakers for the heat strips trip
What affects the repair cost
A thermostat setting costs nothing to fix. A defrost sensor or control board is a moderate repair. A reversing valve replacement is more involved because it requires recovering refrigerant and brazing, and refrigerant leak repair depends on location and refrigerant type. On older heat pumps, a technician can help compare repair against replacement.
