Why Your Air Conditioner Is Dripping Water (and How to Stop It)

On a humid Bangkok night a single bedroom air conditioner pulls a litre or more of water out of the air and sends it down a small plastic pipe to the outside. When that pipe is blocked, the water has only one other place to go: out of the front of the unit, onto the wall, the bed or the floor.

Dripping is the most common non-cooling call we get, and it is almost never a broken machine.

How the water is supposed to leave

The indoor coil runs at around 8–10 °C. Room air at 27 °C and 70% humidity hits it and condenses, the way a cold glass sweats. That water runs down the coil fins into a plastic tray — the drain pan — and out through a 16 mm drain hose that runs alongside the copper pipes to the outside, where it drips onto your balcony or into a downpipe.

For this to work, the pan must be clear, the hose must run downhill all the way, and the coil must not be freezing.

Cause 1: the drain line is blocked

This is the one, eight times out of ten. The water leaving the coil carries dust, dead skin and the biofilm that grows on a wet coil in Bangkok. Over months that sludge settles in the pan and the first bend of the drain hose, algae grows in it, and eventually the hose is blocked. The pan fills, overflows at the front, and the unit drips from the louvre.

Fix: flush the drain line and pan. We do this with water and, where needed, a pump; a wet-and-dry vacuum on the outdoor end works too. This is included in every standard clean, and on a clean unit the test is simple — pour water into the pan and watch it come out the other end.

Why it comes back: if the drain is cleared but the coil above it is not cleaned, the coil keeps shedding sludge into a clean pan. Clearing the drain alone buys a few weeks. Cleaning the coil buys six months.

Cause 2: the coil is dirty enough to overflow the pan

A thick layer of dust on the coil makes condensate run off in the wrong places. Instead of dripping neatly into the pan, water tracks along the dirt, drops off the front edge of the coil and misses the tray. You get dripping even though the drain is clear.

Fix: a proper coil and blower wash. If the coil has visible black mould rather than grey dust, it needs a deep chemical wash rather than a standard clean.

Cause 3: the unit or the pipe was installed without fall

Water only runs downhill. If the indoor unit was hung slightly high on the drain side, or the drain hose sags where it runs through the wall and creates a low point, water pools and backs up. This is an installation fault and it shows up in the first rainy season after fitting, or after someone re-routes the pipes during a renovation.

Signs: dripping from one end of the unit only, or water running down the wall behind it rather than from the front. The fix is to re-hang the unit level or re-run the drain with continuous fall — a repair visit job, quoted in writing after the diagnostic.

Cause 4: the coil is freezing

If ice forms on the coil, it melts when the unit cycles off and the melt overwhelms the pan. Ice means either low refrigerant from a leak, or airflow so restricted — filthy filter, dead blower — that the coil gets too cold. You'll see frost on the copper pipe at the outdoor unit, or hear the fan running but feel very little air.

Fix: clean the unit first if airflow is the problem. If the pressures are low, that is a leak to find and repair, not a top-up. More on the difference in AC not cold after cleaning.

What to do tonight

1. Switch the unit off at the remote and, if water is near the electrical cover, at the breaker.
2. Put a towel down and note where the water is coming from — front louvre, one end, or the wall behind.
3. Check the outdoor drain outlet. If nothing has dripped from it all evening while the unit was running, the line is blocked.
4. Book a clean. Tell us which of the three places the water came from; it changes what we bring.

Running a dripping unit for a week to "see if it stops" is how a ฿690 clean becomes a ฿6,000 control board and a repainted wall.