White frost on the outdoor coil while a UK split is heating, followed by a melt and water from the base, is normal defrost in damp winter air. A solid block of ice that remains, a fan that will not turn, or ice while you are trying to cool, is not normal. Do not chip the ice, do not attack it with a kettle, and do not box the unit in. Keep snow and leaves off the grille from the ground with the fan stopped. If ice survives a defrost or the unit stops heating, call an F-gas engineer. Do not open the refrigerant circuit.
White frost that melts and drains from the base
In heating the outdoor coil is the cold coil. UK winter air is often damp, so frost forms. The system defrosts by warming that coil briefly. The indoor fan may pause, the outdoor fan may change, and clear water should leave the base pan through the drain holes. A light white coat that disappears is the design working. Energy Saving Trust describes the heating mode of these units as an air-to-air heat pump: the outdoor coil is where that heat is collected, and frost is part of a damp winter.
The base must be able to shed that water. Units sat tight on decking, or with drain holes buried in silt, refreeze into a block. A slight lift on proper feet, done at install, matters more than any spray you can buy. Note whether the condenser sits on a wall bracket, a flat roof or the ground, and whether water can leave the base.
Snow and leaves from the ground, then one defrost
From the ground, with the isolator off and the fan stationary, lift loose snow, leaves and carrier bags off the outside of the grille. Do not stand on the unit. Switch the isolator back on only when you are clear of the fan. Ask for heat again and watch one defrost. If the ice softens and drains, the unit is likely fine. Light frost in heating can be left to defrost.
Frequently asked questions
Should I switch the unit off when I see frost?
Light frost in heating can be left to defrost. Switch off if ice is solid, the fan is jammed, you are in cooling, or water is getting into electrics. Do not keep resetting it.
Is outdoor ice a sign the gas has leaked out?
Not on its own in heating. Normal frost is part of defrost. Ice that never clears, or ice in cooling, needs an F-gas engineer to test properly. Do not add refrigerant. GOV.UK certification is the bar for that refrigerant work.
Can I put a box over the outdoor unit in snow?
No. The coil needs outdoor air. A cover used for storage is only for a unit that is switched off and isolated for a long idle period, and even then it must come off before anyone runs it. Do not cover the unit with a sheet while it is running.
Can I pour boiling water over the fins?
No. Boiling water can crack fins and push water into the electrical section, which then trips the breaker. Do not use a sharp tool between the fins. Leave valves and flare nuts untouched.
The outdoor unit is icing while I am in cooling. Is that defrost?
No. If you are in cooling mode and the outdoor unit is icing, that is not winter defrost. Stop and get help. Ice on the indoor unit at the same time is also a fault visit, not a normal heating defrost.
Can I chip ice off a condenser on a high bracket?
No. HSE work at height rules apply if the condenser is on a high bracket. Do not climb a makeshift ladder to chip ice. Do not drill new holes in the base pan yourself. If the fan blade is locked in ice, heating never returns, or ice encases the coil after defrost should have finished, call an F-gas engineer. Causes can include a blocked drain pan, a damaged fan, a sensor fault, or a refrigerant problem. You cannot separate those from the patio.
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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.