In heating mode a split collects heat from outdoor air. On a cold, damp British day the outdoor coil runs below freezing and frost builds on it. A defrost cycle briefly changes how the system runs so that frost can melt. Indoors you may feel the air go less warm, or the fan pause, for a few minutes. That is ordinary winter behaviour. Very long or very frequent defrosts are a reason to book a certified engineer, not a reason to start opening the refrigerant circuit yourself.
Fog, sleet and a thin white coat
The outdoor coil is cold because it is taking heat from the air. Moisture in fog, sleet and a raw 2C morning freezes on the fins. A thin white coat is expected. A solid block that stops the fan is not. Leaves packed into the base hold water and make the icing worse. Cooling mode in summer does not defrost the outdoor coil in this way. If you only use the system in heatwaves you may never see a defrost. Households that heat with the split from October will see it often. Energy Saving Trust's air-to-air pages describe that heating behaviour. Keep the outdoor coil clear of leaves and laundry steam. Give the unit the clearance the manual asks for. Do not box it in to hide it.
Steam, a flooded tray and a few cool minutes
The indoor fan may stop or the supply air may feel cool for a short spell while the coil outdoors clears. A well specified unit then returns to heating. Inverter models usually handle this with less drama than old fixed-speed machines, but they do not abolish it. SCOP already assumes winter operation. It does not promise uninterrupted warm air every minute of a freezing fog. Steam rising off the outdoor unit as the frost melts can look like a fault. It is usually just water meeting cold air. The drain path under the unit matters: a flooded tray refreezes and grows upward into the fan. Ordinary behaviour is a defrost every so often in damp cold, a few minutes each time, heat returning, water dripping away, and airflow through the outdoor coil still possible. A north wall and a sheltered side passage will defrost more than an open, breezy position. If the house has no other heat, ask at survey how this model behaves on a design winter morning and what backup you want. Anti-vibration feet and a legal position still matter, because a noisy defrost at 6am is a neighbour issue as well as a comfort one. The Planning Portal page on air source heat pumps is the heating-plant starting point for that position.
Frequently asked questions
Should I switch the unit off when it defrosts?
No. Let the cycle finish. Switching off mid-defrost leaves frost in place. Use the normal controls and call an engineer if defrosts become constant.
Is defrost a sign of low refrigerant?
Not by itself. Frost in heating mode is normal. Ice that never clears, or heating that has become poor, might involve the charge or the airflow. A certified engineer diagnoses it. Do not add refrigerant yourself.
Can I hose the outdoor unit to remove ice?
Do not chip ice off the fins or pour hot water over the electrical parts. Clear loose leaves when the unit is off and isolated by the normal switch if you can do that safely. Persistent ice is an installer visit.
When is a defrost worth a call?
Call if defrosts seem to occupy more time than heating, if ice does not clear, if a fan scrapes, or if heat never recovers. Those symptoms can be airflow, a drain, a control issue or a refrigerant problem. The person who investigates the refrigerant side needs F-gas certification. GOV.UK publishes that certification. There is no householder procedure for forcing a defrost or for touching the charge.
What should I tell the installer?
Note how long a defrost lasts and the outdoor temperature, and say whether this is new. Water should drain from the outdoor base afterward. A permanent ice block needs an engineer.
Does cooling mode defrost the outdoor coil?
Cooling mode in summer does not defrost the outdoor coil in this way. If you only use the system in heatwaves you may never see a defrost. Households that heat with the split from October will see it often.
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How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.