First cold morning with a new heat pump, you'll probably look out the window and see the outdoor unit putting out steam. The fan's stopped, there's a hiss or whoosh, and the indoor unit might quit blowing warm air. Scares you the first time. Almost always it's fine.
What's going on
In heating mode the outdoor coil is the cold side. It pulls heat out of winter air, so it runs colder than the air around it. When it's near freezing and humid, moisture in the air freezes onto the coil as frost.
Frost blocks airflow, so the heat pump runs a defrost cycle every so often. It briefly switches to cooling mode, which sends hot refrigerant to the outdoor coil. The outdoor fan shuts off so the heat stays put. The frost melts, and that warm wet coil steams in the cold air.
Normal defrost looks like this
- Outdoor fan stops while the compressor keeps running.
- Steam comes off the coil, water drips out the bottom.
- You hear a whoosh when it switches over, and again when it switches back.
- Indoor unit pauses or blows cool air for a few minutes. Lots of mini splits show a defrost light or code.
- It's done in a few minutes, usually under ten.
When it's not normal
- The whole coil's encased in solid ice, not light frost, and it doesn't clear after defrost.
- Ice builds up on the unit top or the fan grille.
- The unit's stuck in defrost, or cycles into it every few minutes.
- It's defrosting on dry days well above freezing.
Heavy ice usually means a drainage or placement problem first. Water from a roof edge or gutter dripping on the unit will freeze there. So will meltwater that can't drain because the unit sits right in snow. A stand or wall bracket that gets it above the typical snow line fixes most of that.
If placement checks out, the cause could be a bad defrost sensor or control board, or low refrigerant. Time to call for service.
Don't chip ice off the coil. The fins and tubes are thin and puncture easy. Turn the system off and let the ice melt, or rinse it gently with warm, not hot, water from a hose.