Drips from a UK ceiling under a cassette, a ducted indoor unit, or the flat above are urgent because plaster soaks quietly and then fails. If the stain grows only when the air conditioner runs, treat it as condensate until proved otherwise: switch cooling off, warn anyone below, and do not keep resetting a pump alarm. If it drips in heavy rain with the system off, the roof, a balcony or a bathroom above may be the source. You can note the timing and catch water. Do not open the ceiling, do not rod drains upwards into a coil, and do not open refrigerant pipes. An F-gas engineer should see an air conditioning leak. A roofer or plumber sees a building leak. Oily drips are an F-gas emergency, not a roof tile.
Water hidden above a cassette or a loft duct
A ceiling cassette in a home office or a small shop dumps condensate into a pan above the tiles. A lift pump then pushes it to a soil pipe or outside. When the pump fails, the safety float should stop the unit. If that float is bypassed or the drain above the ceiling is split, the first sign is a brown ring in the plasterboard, often over a desk.
Ducted units do the same along a loft run. UK lofts freeze in winter and cook in summer. A drain with no fall, missing insulation, or a trap that has dried out will leak onto the ceiling of the room below. High-wall splits usually leak down the wall instead, but the pipe run in the ceiling void can still wet a hallway. Energy Saving Trust's air-to-air description is about moving heat. The water on the ceiling is condensate from that cooling, or it is the building. HSE work at height rules are why a loft or a high cassette is not a casual climb to poke the drain.
Rain with the unit off, or drips only when it cools
Leave the air conditioner off for a wet day. If the ceiling still drips in rain only, tell a roofer or whoever looks after the block. If the ceiling stays dry until someone switches cooling on, stop using cooling. Mop and place a bucket. Look at a visible outdoor spout from the ground: a dry spout while the ceiling weeps means water is leaving indoors. Clear water is condensate. For water that tracks the air conditioner, book an engineer who works on these systems and is certified for F-gas equipment if the indoor unit has to be opened. GOV.UK sets that certification out. Ask them to test the pump, the float and the drain route. Tell a managing agent the same day if the stain is in a leasehold block. Delay is how a pump fault becomes a claim from the flat below.
Frequently asked questions
Can I keep the cassette running if I empty a bucket?
No. The water you see has already left the drain path. Running it adds more water to the ceiling void. Switch off and get the pump or drain repaired.
How do I know it is not the shower above?
Bathroom leaks usually follow showers, not the air conditioning timetable. If the stain grows only when the cassette runs, and stays dry through a shower, the air conditioning drain is the prime suspect.
Should the ceiling be opened before the engineer arrives?
No. Opening it dumps trapped water and can hit wiring. Do not lift ceiling tiles under a cassette while the unit could still be full of water. A tile full of condensate is heavy and the electrics are up there. The engineer needs access, but that is part of the visit, with the unit off.
What if the drip is oily?
If the liquid is oily, or cooling has failed as well as the ceiling, leave the isolator off if you can do that safely. That is refrigerant oil until proved otherwise, and it needs an F-gas engineer rather than a plasterer. Do not open refrigerant pipes. Oily drips are not a roof tile.
Can I drill a relief hole or rod the loft drain?
No. Do not drill a relief hole in the ceiling. Do not rod drains upwards into a coil. Do not pour bleach into a loft drain you cannot see.
The ceiling drips in heavy rain with the system off. Who attends?
If it drips in heavy rain with the system off, the roof, a balcony or a bathroom above may be the source. A roofer or plumber sees a building leak. Keep an F-gas engineer for water that appears when the air conditioner runs.
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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.