On a cold, damp morning, a heat pump can do things that look alarming the first time you see them: frost on the outdoor coil, the fan stopping, a whoosh, and a cloud of steam. Most of the time, that's the heat pump doing exactly what it's designed to do.
Why a heat pump frosts up
In heating mode, a heat pump pulls heat out of the outdoor air. That makes its outdoor coil colder than the air around it, and on a cool, humid morning, moisture in the air freezes onto the coil, the same way frost forms on a car windshield. A light coat of frost is normal.
What defrost mode looks like
Frost blocks airflow, so every so often the heat pump clears it. It briefly runs in reverse to warm the outdoor coil. While it does:
- The outdoor fan stops.
- You may hear a whoosh or a click as the system switches over, and again when it switches back.
- Steam rises off the outdoor unit as the frost melts. It's water vapor, not smoke.
- Water runs out from under the unit.
- Indoors, the air may feel a little cooler for a few minutes, or the indoor fan may pause.
A defrost cycle usually takes a few minutes, and the heat pump goes back to heating on its own.
When it's a problem
- The whole unit is encased in ice, including the top or the sides, not just a light frost on the coil.
- The ice doesn't clear after defrost cycles, or keeps building through the day.
- The house isn't keeping warm.
- The fan is iced in or scraping.
- "Steam" smells like burning, or looks like smoke. Turn it off and call. See heat pump smells.
Heavy ice can come from a defrost control or sensor that's failed, low refrigerant, a fan problem, or water dripping onto the unit from a roof or gutter above.
What you can do
- Keep the outdoor unit clear of leaves, plants and anything leaning against it.
- Make sure gutters or roof runoff aren't dripping onto it.
- Never chip ice off the coil with anything sharp. The fins and tubes are thin and easy to puncture. Pouring warm (not boiling) water over heavy ice can help in a pinch, with the system turned off.
If it's iced over and staying that way, call us at 805-965-6326 or send us a message.
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