Air Conditioning Leaking Water Indoors: Causes, Emergency Steps and Costs
A wall unit that drips or leaves a tide-mark has one problem: the water it pulls out of the air is not getting outside. The five causes, what to do in the next ten minutes, what the repair costs, and how an annual service stops it happening again.
Where the water comes from
In cooling mode the indoor coil runs below the dew point, so moisture condenses on it — litres a day in muggy weather. It collects in a tray and leaves by gravity through a small-bore drain to outside, or via a condensate pump where gravity is not possible. A leak indoors means that path has failed between tray and outlet.
1. Blocked condensate drain
The usual suspect. The condensate pipe is typically only about 16mm bore, and a narrow pipe in the dark with a constant trickle of water is perfect for algae, the pinkish biofilm you get in wet fittings, and the dust that slips past the filters; insects and leaves clog outdoor terminations. Partially blocked, the tray overflows on hot, humid days then behaves for a while, which is why the fault seems to come and go. Fully blocked, it leaks every time it runs. A quick tell is to check the outdoor drain stub on a warm day — if the room feels muggy but nothing is dripping outside, the water is backing up inside instead.
2. Failed condensate pump
Units without a downhill route outside use a small pump with a float switch. When the pump dies or the float sticks, the reservoir fills and spills; some units shut down on a float alarm and show an error code instead. Failing pumps gurgle or hum, and a blocked pump outlet behaves like a blocked drain.
3. Frozen coil thawing
A coil that has iced up through dirty filters or low refrigerant holds far more water than a tray is designed for. When the unit stops and the ice melts, it floods the tray and runs out of the casing. The leak is the symptom; the cause is airflow or a refrigerant leak — see air conditioning not cooling.
4. Installation pitch and pipe routing
Gravity drains need a continuous fall. A pipe that rises over a joist, sags into a belly or was clipped level holds water and air-locks; the symptom often appears months after installation once a little sludge forms. This is an installation fault, corrected by re-routing rather than repeated unblocking.
5. Cracked, mis-seated or dirty drip tray
Trays crack with age or careless cleaning, and a unit knocked or refitted after a service may sit with the tray out of level so water escapes at the low corner. Replacement trays are brand-specific parts.
Emergency steps in the first ten minutes
- Switch the unit off at the remote. If water is tracking down the wall towards a socket or the isolator, switch off at the isolator or consumer unit as well.
- Catch and contain. A tray or towels under the unit; move electronics, rugs and anything below it. Water runs inside walls, so check the room below if there is one.
- Look at the filters. If they are filthy and there is frost or ice visible, the cause is a freeze — leave it off to thaw fully (two to three hours) and clean the filters before the engineer arrives.
- Check the outdoor drain stub if you can reach it: dry on a hot day confirms a blockage or failed pump.
- Do not restart it to "see if it has stopped". Every cycle adds more water.
- Note the pattern (start-up, after an hour, humid days only) and book an engineer, telling them the brand and whether the unit has a pump. Most drain faults are fixed in one visit.
What a water leak repair costs (2026)
Most leaks are cured inside a standard call-out. Parts are modest; the expensive outcomes are the ones left until the ceiling below is stained.
| Cause | Typical fix | Typical cost |
|---|---|---|
| Blocked condensate drain | Flush and vacuum drain, clean tray, treat against regrowth | £80–£150 call-out |
| Failed condensate pump | Replace pump and float switch | £80–£150 + part (usually £80–£250 all-in) |
| Frozen coil thawing | Clean filters/coil; if it refreezes, leak repair + re-gas | £80–£150 · leak + re-gas £100–£300 |
| Poor drain pitch / air lock | Re-route or re-clip drain, add trap or vent | Call-out + labour, quoted |
| Cracked or mis-seated tray | Re-seat or replace tray | £80–£150 + brand part |
Baselines: call-out £80–£150; re-gas £100–£300; service £75–£150; annual contracts £150–£300. See repair costs and servicing costs.
Servicing is the cure, not the unblocking
A leak cleared but not prevented will be back next summer. A competent annual service flushes the tray and drain, doses against algae where appropriate, tests the pump and float, cleans filters and coil so the unit cannot freeze, and checks the drain still falls. Keeping the tray clean and the drain clear also deals with the musty smell that standing condensate causes, so the job is as much about air quality as it is about leaks. Budget £75–£150 per unit per visit, or £150–£300 a year on a domestic contract. Commercial and multi-unit systems belong on planned maintenance for exactly this reason — one blocked drain above a server room costs far more than the contract. The best time to book that service is late spring, before the first heatwave loads the drain up — engineers get busy fast once the hot weather arrives, and a drain cleared in May is one that will not flood your ceiling in July.
If your unit is new and already leaking, the likely culprit is installation pitch or a pump that was never needed; go back to the installer under warranty before paying anyone else.
Sources & methodology
- Condensate management and drain-fall requirements per manufacturer installation manuals (Daikin, Mitsubishi Electric, Fujitsu, Panasonic, LG, Samsung).
- Service scope reflects typical engineer checklists; refrigerant work only by F-Gas certified persons under the GB F-Gas Regulation.
- Price baselines are 2026 UK figures used across this site: call-out £80–£150, re-gas £100–£300, service £75–£150, contracts £150–£300. Quotes vary by brand, access and parts.
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