Water dripping from your indoor unit: the causes, in order
Contents (6)
The call comes on the third day of a heatwave: water is running down the wall under the indoor unit, or dripping onto the parquet. It looks alarming and it is rarely serious. It is also almost never a refrigerant leak — refrigerant is a gas that escapes invisibly and silently, and if you can see liquid, you are looking at condensation that failed to find the drain.
Where the water is supposed to go
A working split condenses moisture out of the room air on its cold indoor coil, typically one to two litres an hour in muggy weather and rather more in a coastal flat in August. That water runs down the fins into a shallow plastic tray under the coil, out through a hose that leaves with the refrigerant lines, and away through the wall — by gravity, which is why the hose must fall continuously, usually at least one centimetre per metre, from the moment it leaves the tray. Where gravity is impossible, a small condensate pump lifts it into a drain instead.
Every cause of an indoor leak is a variation on one theme: the water was made as usual, and something stopped it leaving.
The diagnostic order
Work through these in sequence. The order reflects how often each one turns out to be the culprit in domestic European installations, and following it saves the call-out charge more often than not.
1. A blocked drain — the usual suspect
Dust drawn through the filters mixes with condensate into a slurry, algae and biofilm colonise the damp hose, and after two or three seasons the bore narrows until the tray overflows at the front edge. The signature is unmistakable: the unit runs fine for twenty to forty minutes, then starts dripping from the bottom of the casing, worse on humid days than on dry hot ones. A faint musty smell and gurgling noises often arrive first.
The fix is to clear the hose from its outdoor end — a wet vacuum sealed around the pipe outlet pulls the plug out in seconds, and this is genuinely a homeowner job when the outlet is reachable. Blowing into the pipe from outside is the low-tech version and works surprisingly often. What does not work is pouring bleach in from the indoor side: it pushes the blockage further in and attacks the tray.
2. A frozen coil that has thawed
If ice forms on the indoor coil, the tray receives a slab of meltwater far faster than the hose can carry it — and often the drip appears after you switch the unit off, which is the clue. Look behind the front flap: a white rime on the fins, weak airflow and air that is cold but scarce all point the same way.
Ice means the coil ran below freezing, and there are only two common reasons. The first is restricted airflow — clogged filters, a filthy coil, furniture blocking the outlet, or a fan speed left permanently on minimum. Clean the filters, run the machine on high fan for an hour, and the problem usually disappears for good. The second is a low refrigerant charge, and that is not a consumable: a system that is low on gas has a leak. Under EU rules, refrigerant handling is certified work and venting is illegal, so this one belongs to an engineer — the background is in our guide to the F-gas Regulation.
3. The installation itself
If the unit has leaked since the week it was fitted, stop looking for a fault and look at the workmanship. The indoor unit must sit dead level or with a hair of fall toward the drain outlet; a wall bracket screwed a few millimetres high on the drain side sends water to the wrong corner of the tray. The hose must never rise after leaving the unit, must not be kinked behind the trunking, and must not end in a loop that holds a plug of standing water. On long horizontal runs through a wall, insufficient fall produces a leak that only appears in very humid weather, when the volume of water finally exceeds what a nearly flat pipe can shift.
4. A condensate pump that has stopped
Flats where the drain cannot fall to an outside wall use a small pump, often hidden in the trunking or behind the casing. Pumps clog with the same biofilm as hoses, and their float switches stick. Many models have a safety contact that cuts the air conditioner rather than letting it flood — so a machine that refuses to start on a hot afternoon, with no other symptom, is worth checking for exactly this before anyone suspects the compressor.
5. Sweating, splashing and cracked trays
Three rarer causes complete the list. Insulation that has slipped or perished on the refrigerant lines lets the cold pipe sweat, and the drip lands somewhere entirely different from the unit — follow the water upward before blaming the tray. In very humid rooms, condensation can form on the discharge louvres themselves when the fan is set very low and the louvres point steeply downward; raising the fan speed and flattening the vane angle stops it. And an old tray can crack or its seal can perish, which is a spare part rather than a repair.
Symptom to cause, at a glance
| What you observe | Most likely cause |
|---|---|
| Drips after 20–40 minutes of running, musty smell | Blocked drain hose |
| Water appears mainly after switching off | Frozen coil thawing |
| Weak airflow, ice visible on the fins | Dirty filters or low refrigerant charge |
| Has leaked since installation | Unit not level, or drain with no fall |
| Unit will not start, no other symptom, pump fitted | Condensate pump or float switch |
| Drip located away from the indoor unit | Missing pipe insulation, sweating line |
| Water pouring from the outdoor unit in winter | Normal defrost in heating mode |
What is not a fault at all
An outdoor unit shedding water in heating mode is behaving correctly: in winter the outdoor coil is the cold one, it frosts, and each defrost cycle releases a rush of water. The only thing to manage is where it lands — a puddle that freezes on a shared stairway is a liability, and a drain kit routing it away is standard practice in northern Europe. Likewise, a portable air conditioner that stops with a full-tank warning has not failed; single-hose portables re-evaporate only part of their condensate and the rest has to be emptied, which is one of the daily annoyances that pushes people toward a fixed split system.
Preventing the next one
Almost every indoor leak is a maintenance leak, and the rhythm that prevents it is short. Wash the filters every few weeks during the cooling season and let them dry completely before refitting. Run the unit on fan-only for an hour at the end of the season so the coil and tray dry out rather than fermenting through the winter. Have the drain checked at the annual service — clearing a hose that is half-blocked takes a professional five minutes and is the cheapest item on the whole visit. The full seasonal calendar, including what is legally required in different member states, is in our guide to air conditioning maintenance in Europe.
One caution about the water itself: condensate that has stood in a warm tray is not clean. It carries dust, microbial growth and whatever the biofilm has been doing all summer, so treat a leak onto flooring as a cleaning job with gloves rather than a puddle of distilled water, and do not let it soak into plasterboard for a week while you wait for an appointment.
Frequently asked questions
Is water leaking from my AC dangerous?
Not electrically, in a correctly installed unit, but water tracking down a wall damages plaster and can reach the electrical enclosure of the indoor unit. Switch the machine off at its isolator, put a towel or tray under it, and diagnose without power on. Persistent damp behind the casing also seeds mould in exactly the place air is blown into your room.
Can I clear a blocked condensate drain myself?
Yes, from the outdoor end, and it is the most useful thing a homeowner can do. A wet vacuum held tightly over the pipe outlet, or a firm blow through it, shifts most blockages. Do not dismantle the indoor unit and do not pour chemicals into the tray — the plastic and the seals are not designed for it.
Does a leak mean I need a refrigerant top-up?
Almost never, and this is where households get sold work they do not need. Visible water is condensate. Refrigerant leaves as a gas and shows itself as ice on the coil, poor cooling and higher running cost — and if it truly is low, the system needs the leak found and repaired, not simply refilled, which is both bad practice and, when gas is knowingly vented, illegal in the EU.
Why does it only leak on humid days?
Because that is when the unit makes the most water. A drain that is partly blocked or has too little fall copes with half a litre an hour and overflows at two. Intermittent leaking that tracks the weather is the classic signature of a marginal drain rather than a broken one — and it will get worse every season until it is cleared.
How much water should a split produce normally?
Roughly one to two litres an hour while cooling in humid conditions, all of it appearing at the outdoor end of the drain. A steady trickle outside is a sign of a healthy machine doing its dehumidification work. Nothing at all coming out on a muggy day, with the unit running hard, is a warning that the water is going somewhere else.
Sources
Read next
- Air conditioning that has stopped cooling: the European diagnosis order
- The carbon footprint of cooling a European home
- Cooling a European home without air conditioning: the order that works
- Wall, floor console, cassette or ducted: choosing the indoor unit
- Heating with your air conditioner through a European winter