Heat pump "aux heat" during a freeze: normal or a problem?
Last updated September 2026
Quick answer
Usually normal. When it's very cold outside, or when the house is catching up after you raise the thermostat, a heat pump turns on auxiliary (backup) heat, which is usually electric heat strips. It becomes a problem if aux heat runs all the time in mild weather or if "emergency heat" was switched on by mistake, because backup heat uses far more electricity.
What "aux heat" means
A heat pump moves heat from the outside air into your home. As outdoor temperatures drop, it has less heat to work with, so the system adds backup heat, usually electric strips in the indoor unit. The thermostat shows "aux heat" when that backup is on.
When it's normal
During a freeze, early on a cold morning, while the outdoor unit is defrosting, or right after you raise the thermostat several degrees. In these cases aux heat turns on for a while and then cycles off.
Signs of a problem
Aux heat runs nonstop when it's mild outside, the house never warms up, the outdoor unit isn't running at all, or your electric bill jumps sharply. These can point to a failing heat pump, low refrigerant, a bad sensor or a thermostat setting issue. Call a licensed company to check it.
Defrost mode is normal too
In cold, damp weather, frost builds up on the outdoor coil. The heat pump switches into defrost mode for a few minutes: the outdoor fan may stop and you may see steam rising. That's normal. Ice that keeps building up and never clears is not.
Tips for Houston freezes
Raise the thermostat a couple of degrees at a time instead of cranking it up, which keeps aux heat from running longer than needed. Keep the outdoor unit clear of ice, leaves and anything piled against it. Use "emergency heat" only if the outdoor unit has stopped working, and call for service.
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Heat pump "aux heat" during a freeze: normal or a problem: common questions
What is the difference between aux heat and emergency heat?
Aux heat turns on automatically to help the heat pump. Emergency heat is a setting you choose that shuts off the heat pump and runs only the backup heat. It's meant for when the outdoor unit is broken, and it costs much more to run.
Why is steam coming from my outdoor unit?
It's most likely defrost mode melting frost off the coil. It should stop after a few minutes.
Should I cover my outdoor unit during a freeze?
Don't wrap a heat pump while it's running; it needs airflow to work. Follow the manufacturer's guidance, and keep the area around the unit clear.