Heat pumps are everywhere in North Alabama, and for good reason. Our winters are mild most of the time. Huntsville’s normal January low is about 33 degrees, according to the National Weather Service. Still, a heat pump behaves differently from a furnace, and some of what it does in winter looks alarming when it is perfectly normal. Here is what to expect.
How a heat pump heats
A heat pump does not burn anything. It pulls heat out of the outdoor air and moves it inside. Even 30-degree air holds heat it can use. As it gets colder, there is less heat to move, so the system runs longer. On the coldest days, backup electric heat strips help out.
Normal: steam and a whoosh (defrost)
In cold, damp weather, frost builds up on the outdoor coil. Every so often the heat pump switches into defrost for a few minutes to melt it off. During defrost:
- The outdoor fan stops and you may see steam rising from the unit
- You may hear a whoosh when it switches in and out of defrost
- Water drips under the unit, and can freeze into a small ice patch
- Air at the vents may feel cooler for a few minutes
Most defrost cycles last about 5 to 15 minutes. The steam is just melting frost, not smoke.
Normal: running for long stretches
When it is in the 20s or low 30s, a heat pump may run almost nonstop. That is how it is designed to work: long, steady cycles at lower temperature air. Heat pump air feels warm, not hot like a furnace, often in the 90s at the vent.
Not normal
- Thick ice covering the whole outdoor coil that does not clear after defrost
- An outdoor fan that does not spin, or a unit that hums but will not start
- Running nonstop in mild weather (40s and 50s) without reaching the set temperature
- The “Aux Heat” light on most of the time
- Cool air at the vents for more than 15 to 20 minutes
- Grinding, banging or a burning smell
If you see these, our guide to a heat pump not heating explains the causes.
Aux heat vs. emergency heat
Aux heat (auxiliary) is automatic. The thermostat turns on the electric heat strips to help the heat pump when it is very cold, during defrost, or when you raise the temperature a lot at once.
Emergency heat is a manual setting. It shuts the heat pump off and heats the house with the electric strips alone. Use it only when the heat pump itself is broken, for example if the outdoor unit is frozen solid or damaged, until a technician can repair it. Electric strip heat costs much more to run, so do not leave it on.
Why is my power bill so high with a heat pump?
The usual reason is the heat strips running more than they should. Common causes:
- Big thermostat setbacks. Raising the temperature 5 or more degrees in the morning often brings on the heat strips. The U.S. Department of Energy notes that setting back a heat pump can cancel out the savings. Use small setbacks of 2 to 3 degrees, or a thermostat made for heat pumps.
- Emergency heat left on by mistake
- A dirty filter or dirty coil
- Low refrigerant or a defrost problem
- A long cold spell
Our guide to thermostat settings and high bills has more ways to save.
Winter tips
- Keep the outdoor unit clear of leaves, snow and ice. Leave about 2 feet open around it.
- Make sure gutters do not drip onto the unit. That is a common cause of ice buildup.
- Do not chip ice off the coil. Call instead.
- Change the filter every 1 to 3 months.
- Schedule a fall check. A heat pump works in both seasons, so it should be serviced twice a year.
Call North Alabama Heating & A/C at (256) 534-2941. We have served Huntsville, Madison and North Alabama since 1977, Monday to Saturday, 7 AM to 7 PM. See our heat pump repair service.
Quick answers
Is it normal for my heat pump to run constantly in winter?
Yes, when it is very cold. Below about freezing, a heat pump may run almost nonstop with backup heat helping. It is not normal if it runs nonstop in mild weather or cannot reach the set temperature.
Is it normal for a heat pump to frost over?
Light frost is normal in cold, damp weather. The unit should melt it off in a defrost cycle that lasts about 5 to 15 minutes. Thick ice that does not clear is not normal and needs service.
When should I use emergency heat?
Only when the heat pump itself is not working, such as an outdoor unit frozen solid or damaged. Emergency heat uses electric heat strips alone, which costs much more to run. Call for repair and switch back once it is fixed.
