Heat pumps move heat instead of making it, which is why people worry about them in cold weather. Sacramento does get freezing mornings, but winters here are mild compared with much of the country, and a heat pump that is sized and maintained properly handles them. Here is what to expect and what to watch for.
Do heat pumps still heat in freezing weather?
Yes. A heat pump can still pull heat from outdoor air below 32°F, though it delivers less heat as the temperature drops. The Department of Energy’s cold-climate heat pump program focuses on equipment that keeps its capacity and efficiency at outdoor temperatures below 32°F, and many modern models are built with that in mind. For the coldest hours, most systems have a backup heat source such as electric resistance strips, or a gas furnace in a dual fuel setup.
Frost on the outdoor unit is normal
When the outdoor coil gets colder than the air’s dew point and the air is damp, frost forms on it. Trane calls periodic frost or ice on the outdoor coil perfectly normal. The system has a defrost control that uses an outdoor temperature sensor and a coil sensor to decide when to run a defrost cycle. For more, see heat pump defrost mode.
What happens during defrost
- The system briefly reverses, sending warm refrigerant to the outdoor coil to melt the frost.
- The outdoor fan usually stops.
- Steam can rise from the unit as the frost melts, which is normal.
- Backup heat may run so the house does not cool off.
- A typical cycle lasts about 5 to 10 minutes, according to Carrier.
If defrost cycles are unusually frequent or long, Lennox says that may indicate a problem. A thermostat that blows cool air for longer than a few minutes is another sign. See heat pump not heating.
When ice is a problem
- Ice thicker than about a quarter inch. Carrier advises shutting the system off and calling a professional.
- Ice that does not melt between defrost cycles. Trane says ice that stays points to low refrigerant or a faulty defrost control or sensor.
- The whole coil encased in ice. Do not run the system. Call for service.
Never chip ice off the coil, because the fins and tubes are easy to damage. Our guide on heat pump freezing up covers the causes in detail.
Backup heat and your bill
Backup or auxiliary heat is automatic, and it runs when it is too cold for the heat pump alone, after a big thermostat increase, or during defrost. Emergency heat is manual and should be used only if the heat pump has failed. Backup heat costs much more to run than the heat pump itself. The Department of Energy advises against manually setting back a heat pump thermostat if it causes the backup heat to come on. See our thermostat settings guide and heat pump high electric bill.
What you can do before winter
- Change the filter, and check it monthly. The Department of Energy advises changing heat pump filters once a month or as needed.
- Clear leaves and debris from around the outdoor unit.
- Make sure snow, ice or a cover is not blocking it.
- Book a yearly inspection. See how often a heat pump needs service.
Heat pump or furnace?
For many Sacramento homes a heat pump is a practical option because winters are short, and a single system provides both heating and cooling. SMUD customers can also find rebates for qualifying heat pumps. Our heat pump vs furnace guide covers the trade-offs, and our heat pump services page describes what we do.
We service heat pumps in Roseville, Rocklin, Folsom and Elk Grove. If yours is not keeping up, contact us.
Frequently asked questions
Do heat pumps work when it is below freezing?
Yes, though output and efficiency drop as it gets colder. The Department of Energy runs a cold-climate heat pump program aimed at capacity and efficiency below 32°F. Many heat pumps also have electric backup heat for the coldest hours.
Is frost on the outdoor unit normal?
Yes. Trane says periodic frost or ice on the outdoor coil is perfectly normal, and the system melts it with a defrost cycle. Frost that stays, or ice that builds into a thick layer, is not normal.
Is steam coming from my heat pump a problem?
Usually not. During defrost the unit reverses and warms the outdoor coil, and the melting frost can look like steam. It should stop when the cycle ends. Call for service if the cycles are unusually frequent or long.
How long does a heat pump defrost cycle last?
Carrier says a typical defrost cycle lasts about 5 to 10 minutes. Other manufacturers give similar short windows. During defrost the outdoor fan stops and backup heat may come on.
What should I do if ice builds up on my heat pump?
Carrier advises shutting the system off and calling a professional if ice is thicker than about a quarter inch. Never chip the ice off, because that can damage the coil.