The short answer
In 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.
On this page
A cloud of steam rising from the outdoor unit on a cold morning, with the indoor unit gone quiet, can look like a fault. Usually it is the system clearing ice from its outdoor coil. Any heat pump that takes its heat from outdoor air has to do this in the right conditions.
Why ice forms on the outdoor coil
To collect heat, the refrigerant in the outdoor coil has to be colder than the air passing over it, often by several degrees. When the fins are colder than the air's dew point, moisture condenses on them, and when they are below 0°C that moisture freezes into frost.
Frost acts as insulation and gradually blocks the gaps between the fins. Less air gets through, less heat is collected, and if nothing cleared it the system's output and efficiency would keep falling.
How much frost builds up depends as much on moisture as on temperature:
| Weather | Frost on the coil |
|---|---|
| Mild and dry | Little or none |
| Damp, around freezing or a few degrees above | Builds up fastest |
| Very cold and dry | Builds up more slowly |
| Freezing fog or wet snow | Can build up quickly |
Very cold air holds little moisture, so a hard, dry frost outside can mean fewer defrosts than a raw, drizzly day just above freezing, which is common in a UK winter.
How the system clears the ice
The controls decide when to defrost, typically using the outdoor coil temperature, the outdoor air temperature and the time since the last defrost. In most wall-mounted systems a defrost then works like this:
- The indoor fan stops, so cold air isn't blown into the room.
- The reversing valve switches the system into its cooling direction, sending hot gas from the compressor to the outdoor coil.
- The outdoor fan stops, so the heat stays in the coil and melts the ice.
- Once the coil is clear, the valve switches back and the outdoor fan restarts.
- The indoor fan waits until the indoor coil is warm again, then heating resumes.
A defrost usually takes a few minutes. How often one is needed depends on the weather: in the worst conditions it can come round many times a day, while in cold, dry weather it is needed far less. Some of the heat for melting the ice is drawn from indoors, so the room may cool slightly while it happens. Some outdoor units also have a small heater in the base to stop meltwater refreezing.
What a defrost looks and sounds like
| What you notice | What is happening |
|---|---|
| Indoor unit stops blowing | Heating paused to avoid a cold draught |
| A light flashes or a symbol shows | Some units signal a defrost this way |
| Hiss or whoosh from outside | The reversing valve switching over |
| Outdoor fan stops but the unit hums | Compressor sending heat to the coil |
| Steam from the outdoor unit | Warm, wet fins meeting cold air |
| Water pouring from the base | Melted ice draining away |
When it isn't normal
Defrosting is part of heating with outdoor air. These signs, though, suggest something is wrong:
- Ice that keeps building until the coil is encased, even though defrosts are happening. The defrost may not be working, for example because of a faulty sensor or valve.
- Ice growing up from the base. Meltwater may be refreezing because the drain holes are blocked or the unit sits too low, in snow or standing water.
- Frequent defrosts in mild, dry weather. This can point to a fault, including a refrigerant leak, which makes the coil run colder than it should.
- Ice on the pipes or indoor unit in cooling mode. That is a different problem, covered in ice on the pipes or outdoor unit.
If ice keeps building up, switch the system off and call an engineer rather than trying to clear it yourself.
Helping the defrost do its job
- Give the water somewhere to go. A wall bracket or raised stand lets meltwater drain away and helps keep the unit clear of lying snow. Below it, a drain, gravel or another surface that lets water soak away works well, and the water shouldn't run across a path where it could freeze. See where the water goes: condensate drainage.
- Keep it clear. Gently brush snow and leaves away from around and on top of the unit so air can flow, and never cover it while it is in use.
- Leave the fins alone. Don't chip or scrape ice off the coil, because the thin aluminium fins bend and tear easily.
- Let a defrost finish. Switching the system off and on during a defrost doesn't help and only delays the return of heat.
Seasonal checks are covered in looking after the outdoor unit.
Related guides
- How air conditioning heats a roomTo heat, the system runs its refrigerant loop in reverse. The outdoor coil turns cold and collects heat from outside air, even below freezing, while the indoor coil turns hot and warms the air blown into the room.
- Ice on the pipes or outdoor unitA 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.
- 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.
- Where the water goes: condensate drainageWater that condenses in the indoor unit as it cools runs through a drain hose to a suitable outlet, with a small pump where it can't fall naturally. In heating mode the outdoor unit produces water too.