The larger pipe on a UK split is cold in cooling and can sweat if the insulation is split, crushed in a loft, or missing after decorating. Sweat is damp lagging. Ice is a hard white build-up and means the pipe or coil is too cold. Do not peel insulation off to have a look, do not chip ice, and do not pour hot water. Check filters and airflow, then leave cooling off if ice is already there. An F-gas engineer should find out whether this is damaged lagging, poor airflow, or a refrigerant problem. You should not open the gas circuit either way.
Damp lagging is sweat. A white sleeve is ice
Good insulation on the cold pipe stays dry on the outside. A nick from a cable clip, a loft run where a joist has flattened the armaflex, or UV-perished lagging on an outside run will sweat in humid UK weather and drip inside a cupboard. That is a lagging repair, still best done by someone who will not kink the pipe. A wet patch on the pipe insulation is usually condensation from damaged lagging, not a gas leak by itself.
White ice on the outside of intact insulation, ice at the indoor unit, or ice back to the outdoor valves is beyond sweaty lagging. It often arrives with weak cooling. Low airflow and low refrigerant can both produce it. You cannot tell which by squeezing the pipe. Energy Saving Trust's air-to-air description is the heat move those pipes are doing. It is not a reason to strip the foam. Mention if the pipes were disturbed during a kitchen refit.
Leave the foam on until someone can see it
Photograph the ice, then stop cooling so it can melt into a towel. Fan-only is acceptable if the manual allows and water will not run into a socket. Do not cut the insulation away. The engineer needs to see whether the lagging was the cause. Do not wrap the ice in extra towels tight to the wall and leave cooling running. Wash filters once the ice has gone, before any new test. If the pipe ices again on the next cool run, stop. Do not add layers of tape as a cure. Tape hides a crushed section and holds water against the copper. Do not peel insulation off to decide. An F-gas engineer should trace airflow, the condition of the insulation, and the operating pressures if the symptoms justify connecting gauges. That is their decision on site. HSE F-gas rules and GOV.UK certification are why you do not back off a flare to let ice melt inside the pipe. Venting refrigerant is not a homeowner step and it will not fix airflow. If ice is only on a loft section where squirrels or a decorator ripped the lagging, say that. The repair may be insulation and support, provided the engineer is satisfied the circuit is sound. If oil is visible where ice was, treat it as a leak and leave the unit off.
Frequently asked questions
Is a wet patch on the pipe insulation a leak of gas?
Usually it is condensation from damaged lagging. Ice, oil, or cooling that has failed needs an F-gas engineer. Do not strip the insulation to decide.
Can I replace the foam tube myself?
Only if the pipe is undamaged, unfrozen, and you can slip the correct closed-cell lagging on without bending the copper or uncovering a joint you then disturb. If the pipe is iced or oily, leave it.
Why did the pipes ice after I closed the bedroom door and shut the louvre?
You may have starved the coil of air. Open the louvre, clean the filter, and do not run it iced. If ice returns with normal airflow, stop and book an engineer.
Should I pour hot water on the ice?
No. Do not pour hot water and do not chip the ice. Stop cooling, let it melt into a towel, and use fan-only only if the manual allows and water will not run into a socket.
Can I vent the pipe so the ice inside melts?
No. Do not back off a flare. Venting refrigerant will not fix airflow, and the gas circuit stays closed. An F-gas engineer decides whether gauges are justified.
The lagging was ripped in the loft during decorating. Is that the whole fault?
It may be a lagging and support repair if the pipe is not iced or oily and the engineer is satisfied the circuit is sound. If oil is visible where ice was, treat it as a leak and leave the unit off. Do not stand on joist edges you do not trust to photograph a loft run.
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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.