Why your heat pump is steaming in winter (and when to worry)

The first cold morning after you get a heat pump, you'll probably look out the window and see the outdoor unit billowing steam. The fan has stopped, there's a hiss or a whoosh, and the indoor unit might stop blowing warm air. It's alarming the first time. It's almost always normal.

What's actually happening

In heating mode the outdoor coil is the cold side of the system. 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 periodically runs a defrost cycle. It briefly switches into cooling mode, which sends hot refrigerant to the outdoor coil. The outdoor fan shuts off so the heat stays on the coil. The frost melts, and the warm, wet coil steams in the cold air.

Normal defrost looks like this

• The outdoor fan stops while the compressor keeps running.
• Steam comes off the coil, and water drips out the bottom.
• You hear a whoosh when it switches over and again when it switches back.
• The indoor unit pauses or blows cool air for a few minutes. Many mini splits show a defrost light or code.
• It's over in a few minutes, usually under ten.

When it's not normal

• The whole coil is encased in solid ice, not a light frost, and it doesn't clear after a defrost cycle.
• Ice builds up on the top of the unit or the fan grille.
• The unit is stuck in defrost, or cycles into it every few minutes.
• It's defrosting on dry days well above freezing.

Heavy ice usually points to a drainage or placement problem first. Water from a roof edge or gutter dripping onto the unit will freeze on it. So will meltwater that can't drain away because the unit sits on the ground in snow. A stand or wall bracket that lifts it above the typical snow line fixes most of that.

If placement isn't the issue, the cause could be a failed defrost sensor or control board, or low refrigerant. That's a service call.