Why Your Heat Pump Ices Over In Winter (And When It Is A Problem)
Heat pumps are everywhere in Washington now, and every winter we field the same call: the outdoor unit is covered in ice, is that normal? Sometimes yes.
Why frost forms at all
In heating mode the outdoor coil is colder than the outside air, because it is pulling heat out of that air. When the air is cold and damp, which describes most of a Washington winter, moisture condenses on the coil and freezes. This is physics, not a fault.
The defrost cycle
Every heat pump handles this by periodically reversing itself, briefly running in cooling mode to warm the outdoor coil and melt the frost. You will see steam rising off the unit, hear a loud whoosh as the reversing valve shifts, and notice the outdoor fan stop. It lasts a few minutes, then the system returns to heating. Backup heat usually kicks in during that window so the house does not get a blast of cold air.
Normal frost is a light coating that clears completely. Normal defrost happens every 30 to 90 minutes in cold damp weather.
When it is a real fault
- The coil is encased in solid ice, not frost, and never clears.
- Ice builds on the top of the unit or on the fan blades.
- The unit never enters defrost even in freezing fog.
- It defrosts constantly, every fifteen minutes, and never heats properly.
- Emergency or auxiliary heat runs continuously.
Those point at a defrost control board, a failed defrost thermostat or sensor, a stuck reversing valve, low refrigerant charge, or a fan motor that is not running. All of them need gauges and a meter to separate.
What you can safely do
Clear snow and leaves from around the unit and keep at least 18 inches around it. Make sure the unit is not sitting in a puddle that freezes up into the coil, which happens when the pad has settled or the drainage is blocked. Never chip ice off with anything metal; you will puncture the coil and turn a defrost fault into a refrigerant leak. If you must, warm water only, and switch the system to emergency heat first.
Getting it sorted
Ice that never clears will eventually damage the fan and the coil, so it is worth a call rather than a wait. Book heat pump repair in your city, or read about repairing versus replacing if the system is nearing the end of its life. You can book in any Washington city — Seattle, Spokane, Tacoma, Vancouver or Yakima — from our service areas page.
Need this fixed rather than explained? Call (888) 380-4427 any hour, or find your city on our service areas page.