Why does my heat pump steam, ice up or drip water?

Updated 29 September 2026

Short answer: it's defrosting. In damp weather just above freezing, ice builds up on the outdoor unit, and the heat pump briefly melts it off. A few minutes of steam is normal.

Damp air around 0 to 5°C is the worst for this, because there's plenty of moisture to freeze. While it defrosts, the heat pump borrows a little heat from the house, so efficiency dips slightly on those days. That's expected, and it's one reason a raw, foggy day can look worse than a dry, colder one.

Some ice on the back of the unit in cold weather, and water dripping from it, are part of the same cycle. The water should run away through the drain pipe; if it pools, check the pipe isn't blocked by leaves. In a hard frost the drain can freeze for a while, and it clears when it warms up.

Lots of ice, ice in mild weather, or water that isn't from the drain are worth reporting to Octopus, as they can mean poor airflow or a leak. Don't try to fix it yourself.