What this symptom may mean
A heat pump moves heat from outdoor air into the house, even on cold days. Its air feels warm but not as hot as a gas furnace's, so it can seem lukewarm to the hand. That is normal if the rooms still reach the setpoint.
In cold, damp weather, frost builds on the outdoor coil, so the system runs a defrost cycle, briefly reversing to warm the coil. Expect a whoosh, some steam and a few minutes of cooler air before heating resumes.
Auxiliary heat is a backup, usually electric heat strips, that switches on by itself when the heat pump needs help. Emergency heat is a setting you choose that runs only that backup, meant for when the outdoor unit fails.
Possible causes
Normal defrost cycle
Steam and a short pause in heat are expected, especially on cold, humid mornings.
Thermostat setup
The thermostat may be in COOL, or emergency heat may have been switched by accident.
Heavy ice on the outdoor coil
If defrost fails, ice can encase the coil and block heat transfer.
Auxiliary heat failure
When backup heat strips fail, the house may fall behind on the coldest days.
Safe checks you can make
- Confirm the thermostat is on HEAT with a setpoint above room temperature, and check batteries.
- Replace the air filter, since heat pumps rely on steady airflow.
- Look at the outdoor unit. Light frost that clears periodically is normal; solid ice for hours is not.
- Clear leaves and debris from around the outdoor unit, and make sure gutters are not dripping onto it.
- Check the indoor and outdoor breakers, resetting a tripped one once at most.
What not to do
- Don't chip ice or pour hot water on the outdoor coil. It can damage the fins and tubing.
- Don't leave the system on emergency heat for days unless a provider recommends it.
- Don't open the outdoor unit or touch any electrical parts.
- Don't keep resetting a breaker that trips again.
When professional evaluation may be appropriate
Call if the outdoor unit stays encased in ice, auxiliary heat runs almost constantly, or the house cannot reach the setpoint on mild days. Defrost, refrigerant or valve trouble is likely.
Call to be connected with an independent provider for heat pump repair in the Montgomery, AL area. Ductless systems have their own guidance on the mini-split page.
- Ice covers the outdoor coil for hours
- Auxiliary heat shows as running nearly all the time
- Cold air from the vents outside of defrost
What an HVAC provider may inspect
- Defrost control board and sensors
- Reversing valve operation
- Refrigerant charge and leak points
- Auxiliary heat strips and sequencers
Keeping it from happening again
Keep the outdoor unit clear of leaves and gutter runoff and change filters on schedule. Heat pumps here run most of the year, so spring and fall checkups help them through hard freezes and long summers alike.
