Air conditioner leaking water from the indoor unit: blocked drain, hose or pump

Short answer: water running down the wall from the indoor unit is almost always condensate that cannot drain away. The drain is blocked with dust and slime, the hose runs uphill somewhere, or the condensate pump has stopped working. It is rarely refrigerant: that is a gas and you will not see it dripping. Switch the unit off, catch the water and have the drain flushed.

Where the water comes from

An air conditioner cools by blowing air over a cold evaporator. The moisture in that air condenses on it, just like on the outside of a cold glass. That water runs into a tray under the evaporator and from there via a hose to the outside or to a drain. In warm, humid weather this can amount to litres a day.

So the whole route is: evaporator, condensate tray, drain hose, and sometimes a small pump. Things can go wrong at any of these points, and the result is always the same: water running over the edge of the tray and coming out of the inside of the unit.

The four causes in order

Blocked drain. By far the most common. Dust gets into the tray and, together with moisture, forms a layer of slime. This slowly washes towards the hose outlet and clogs it. An engineer blows or sucks the pipe clear and rinses the tray; after that the problem is gone straight away.

Incorrectly laid hose. The drain works by gravity. If there is a loop in it or it runs uphill somewhere, water stays in it and silts up. This is a typical installation error that only shows up after a few seasons.

Faulty condensate pump. On units where the drain cannot run downwards by itself, there is a small pump. This pump has a float that monitors the water level. If it sticks or the pump is broken, the tray overflows. Some systems report this with a fault code and switch off cooling as a safety measure.

Ice build-up that thaws. If the evaporator freezes because of dirty filters or too little refrigerant, that ice thaws all at once at some point. The tray cannot cope with that amount and overflows. In that case the water is a symptom and the real problem lies in the cooling itself; read our guide on an air conditioner that no longer cools.

What you can do yourself

Switch the air conditioner off and catch the water; do not let it keep running, as the water will then reach the electronics and run down the wall. Take out the filters and clean them, because dirty filters are the cause behind the cause. Check where the drain hose comes out outside: if nothing is dripping there while the unit has been cooling for hours, the drain is blocked. Also check that the outlet has not become overgrown or occupied by insects.

Flushing the drain is best left to a professional: with too much pressure you blow the hose off the tray, and then it really leaks. The related parts, such as drip trays and drain kits, can be found under condensate traps and condensate drains.

When it really is something else

If the water is not at the indoor unit but at the outdoor unit, that is usually normal: in heating mode it defrosts, and that water is supposed to run out. Do make sure it can drain away and does not freeze on a path.

If you see oily traces at the pipe connections, that may indicate a refrigerant leak; refrigerant carries compressor oil with it. That is a job for a certified refrigerant (F-gas) technician, and it must be repaired, not topped up.

Prevention

Have the indoor unit cleaned every year, including the condensate tray and the drain. That is exactly the work that prevents leaks and also solves odour problems. At the end of the season, run the air conditioner in fan mode for an hour: the evaporator dries out and less moisture remains for mould to grow in.

Blijft een oudere mobiele airco lekken, dan is vervangen vaak voordeliger dan repareren; zie · .

Frequently asked questions

Is the water dangerous?

It is condensate from the room air, so in itself clean. However, it often contains dust and slime from the tray, and it runs down walls and over electronics. Switch the appliance off and do not let it keep leaking.

Why does my air conditioner only leak on hot days?

Because that is when the most condensate is produced. A partially blocked drain can still cope with a small amount, but not at full output. So it is not a coincidence but a sign that the drain is too slow.

Can I clean the condensate tray myself?

On some models you can remove the bottom cover, but on many units the tray sits behind the evaporator and the unit has to be partly dismantled. You can always clean the filters yourself; the rest is maintenance work.

Should there be a trap in the condensate drain?

Yes, for installations that discharge into the sewer; otherwise the sewer sucks the water seal empty or air comes back up the pipe. If the drain discharges freely to the outside, it is not necessary.