Water on the floor under a UK indoor unit means condensate is missing its drain. Gravity pipes blocked by algae or a kink, a failed condensate pump, or a drain with no fall will all dump water indoors. Switch the unit off so it stops making water, catch what you can, and look at the outdoor spout if you can do that from the ground. Do not rod the drain back towards the coil with wire. If a ceiling below is staining, if a pump alarm is sounding, or if water is near electrics, stop and call an engineer. Do not open the refrigerant pipes. Clear water is condensate. Oily water is a different, more urgent visit.
A gravity spout buried in the flower bed
A high-wall split in a house often drains by gravity through a small pipe to an outside spout. If that spout is buried in a flower bed, crushed by a gate, or full of algae, the pan fills and overflows through the indoor unit onto the wall or floor. The clue is water only while cooling, and a spout that stays dry. Stop cooling. Fan-only does not make condensate the way cooling does. Mop up and move furniture. If the outdoor spout is at ground level, clear leaves and silt from the very end.
Pumps in flats and basements
Flats and basement rooms often use a condensate pump because there is no downhill run to outside. The pump sits in a small tank. A float should start the pump and, if the tank keeps rising, stop the unit or raise an alarm. When the pump dies, you get a beep and then water, sometimes into the ceiling of the flat below. Mention the floor and any room below when you book. If you have a pump and you know where the safe service point is because the installer showed you, follow that note only.
Fall, a trap, or a pipe that sweats on the plaster
A third pattern is install geometry: a pipe that runs uphill, a trap in the wrong place, or insulation missing so the pipe sweats onto the plaster. Sweating on the pipe surface can wet a wall even when the spout works. Do not disconnect pipework you cannot refit with the right fall. Check whether a window above or a bathroom leak could be the water. Air conditioning water is cold and appears when the unit runs. A roof leak appears in rain with the unit off. HSE ventilation notes are a reminder that wet indoor air is a building condition, separate from a blocked spout. GOV.UK certification still applies if an F-gas company has to open the indoor unit properly. The gas circuit stays closed for a clear-water drain job.
Frequently asked questions
Is air conditioning water dirty?
Condensate is moisture from room air. It can pick up dust and algae from the pan, so do not drink it and do not assume it is sterile. It should not be oily. Oily residue needs an F-gas engineer. If the liquid is oily, or there is ice on the coil as well as water, leave the isolator off if you can do that safely and do not restart.
Can I drill a new drain hole myself?
No. Fall, traps, insulation and where the water ends up are install decisions. A new hole in the wrong place leaks inside the wall. Do not blow backwards into the drain with a compressor or your mouth. You will push slime into the pan. Do not rod the drain back towards the coil with wire.
Why does it only leak on hot days?
Hot, humid days make more condensate. A drain that is almost blocked copes in mild weather and overflows in a heatwave. That is still a drain fault, not a sign the unit is too powerful.
When does a wet ceiling below become the emergency?
Call an F-gas engineer the same day if water is staining a ceiling, if the pump alarm will not clear, or if the indoor unit has overflowed more than once. A blocked drain is ordinary maintenance. Leaving it overflows plaster, and in flats it becomes a neighbour's ceiling. Do not bypass the float switch with a paperclip.
Could the water be a roof leak instead?
Air conditioning water is cold and appears when the unit runs. A roof leak appears in rain with the unit off. If water is near electrics, or a breaker has tripped, keep the unit off and say so. An electrician joins the visit only in that case. Nobody should open flare nuts to see where water enters.
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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.