It’s 39°F and cloudy in Louisville with humidity at 81%. Near perfect conditions for heavy frost on your heat pump. If your heat pump looks like this, don’t be alarmed. It will clear itself in within the hour as it performs a defrost cycle. Why does frost form on your heat pump? How does this all work?

Frost on your heat pump is okay.

A heat pump is nothing but an air conditioner with an extra couple of components – a reversing valve and a defrost control board. The typical air conditioner rejects heat into the outside air and extracts heat (cools the air) from inside your home. While it’s removing heat from your home, it is also removing moisture because the coil temperature is less than dew point of your home. The same thing is happening in the winter except it is extracting heat from the outdoor air. The surface temperature of your outdoor coil is cold enough that frost forms on the coil.

Have you ever heard the loud ‘swoosh’ from your heat pump? That’s the sound of the reversing valve being energized for a defrost cycle. The outdoor coil is warmed and the frost melts from the coil. This happens every 30-120 minutes depending on your particular brand of heat pump. It’s always best to have the defrost cycle set to the longest interval possible. Some areas of the country have so much moisture in the air that they must defrost every thirty minutes. At my home in near Louisville, my heat pump is set to defrost every 120 minutes. I took this picture today. The heat pump was running and maintaining a nice 70°F with no auxiliary heat.

If the frost continues to build and you notice the AUX heat is energized continuously on your thermostat, then it’s best to call for service. No need for a surprise $400 utility bill!