The short answer
A thin layer of frost on the outdoor unit while it heats in cold, damp weather is normal and clears when it defrosts. Ice that stays or keeps building, or that appears while cooling or on the pipes, points to a fault.
On this page
Ice on an air conditioner looks like a sign that something has gone wrong, but on a system that heats, some frost is part of normal running. What matters is the mode the system is in, where the ice has formed and whether it clears by itself.
Normal frost when heating
To collect heat from outside, the outdoor coil has to run colder than the outdoor air. When the air is cold and damp, typically within a few degrees of freezing, moisture freezes onto the coil as frost. That's expected.
The system detects the build-up and runs a defrost. For a short time it reverses to warm the coil: the indoor fan stops, the outdoor fan usually stops too, and you may hear a whoosh and see clouds of steam and water running from the base. Then heating resumes. On a cold, damp day this can happen several times.
When ice isn't normal
| Where and when | Normal? | Likely reason |
|---|---|---|
| Thin frost on the outdoor coil, heating | Yes | Cold, damp air; clears in a defrost |
| Steam from the outdoor unit, heating | Yes | A defrost melting the frost |
| Ice on the ground below the unit | Common in a freeze | Defrost water refreezing |
| Thick ice that stays on the outdoor unit | No | Defrost, drainage or refrigerant fault |
| Ice on pipes or valves at either unit, cooling | No | Low refrigerant or poor airflow |
| Ice on the indoor unit's coil | No | Dirty filters, fan fault or low refrigerant |
In a long freeze, ice left to build up under the outdoor unit can reach its base, block the drain holes and trap more water, which then freezes onto the coil. Some models have a heater in the base to keep the drain holes clear.
Ice in cooling or dry mode has different causes. Blocked filters or a failing fan let the indoor coil get too cold, and so does a shortage of refrigerant. Running cooling on a cold day, outside the temperature range given in the manual, can also ice the indoor coil.
What you can do
- Check the mode. Ice while cooling or drying isn't part of normal running, so switch cooling off.
- Give a defrost time. In heating mode, keep an eye on the unit for an hour or two. Frost that clears during a defrost needs nothing from you.
- Clean the indoor filters. Blocked filters are a common cause of ice on the indoor unit. See how to clean the filters.
- Thaw indoor ice gently. Leave cooling off, or run fan-only mode, with towels under the unit to catch the meltwater. Water dripping from the indoor unit covers what to do if it overflows.
- Clear snow and ground ice. Keep snow away from the outdoor unit and stop ice building up beneath it, without touching the coil. If defrost water freezes on a path, grit it and mention it to the installer.
What not to do
- Don't chip, scrape or knock ice off the coil or pipes. The fins and pipes are thin, and a puncture lets refrigerant escape.
- Don't pour hot water over the unit. It refreezes in the base and around the drain holes and can make the build-up worse.
- Don't use a heater, hairdryer or heat gun near the unit.
- Don't cover the unit to keep frost off. It needs air to run and to defrost.
When to call an engineer
- Ice that doesn't clear after a defrost, or after the system has been off for a few hours in above-freezing weather.
- Any ice while cooling, or on the indoor unit, once the filters are clean.
- A scraping or banging sound from the outdoor fan, which may be hitting ice. Switch the system off straight away.
- Ice that keeps coming back, especially alongside poor heating or cooling, a hissing sound or an error code.
Refrigerant isn't used up, so a shortage usually means a leak, though it can also mean the system was given the wrong amount when it was installed. Either needs an engineer with the right F-gas qualification, and topping up without finding and fixing a leak doesn't solve anything.
Related guides
- Why the outdoor unit defrosts in cold weatherIn heating mode the outdoor coil runs colder than outdoor air, so in damp weather near or below freezing, moisture freezes onto it. Every so often the system briefly reverses to send heat to the outdoor unit and melt the ice, which is normal.
- Looking after the outdoor unitKeep the space around it clear so air flows freely, brush off leaves and snow with the system switched off, and leave cleaning the coil and anything inside the casing to an engineer. Otherwise it needs little beyond its regular service.
- Using air conditioning to heat in winterChoose heat mode, point the airflow down and give the room time to warm. Most systems keep heating well below freezing, though less efficiently, and pause now and then to defrost the outdoor unit, which is normal.
- When to call an engineerCall an engineer when simple checks don't fix a problem, and switch off straight away for burning smells, smoke, a circuit breaker that keeps tripping or signs of a refrigerant leak. Refrigerant, electrics and anything inside the casing are work for a qualified engineer.