Air Conditioning Condensate And Where The Water Goes

Every air conditioner is also a dehumidifier. On a humid Sydney afternoon a single unit can pull several litres of water out of the air.

That water has to go somewhere, and the pipe that takes it is the most neglected part of the system.

How the water is made

Warm, humid air passes over a cold coil. The moisture in it condenses onto the coil and runs into a tray underneath.

From the tray, it drains away by gravity through a small pipe, or it is lifted by a condensate pump to somewhere it can drain.

Nothing about this is complicated. All of the trouble comes from the drain being small, hidden and permanently damp.

Why the drain blocks

The tray and the pipe stay wet and dark, which is ideal for biological growth. A slime builds up inside the pipe over a season or two and eventually closes it.

Dust from the return air lands on the wet coil, washes into the tray, and mixes with that growth into a sludge.

Insects get into the open end of a drain line and nest in it.

And the line frequently has a sag in it where it was never supported properly, so water sits in the low point and everything settles there.

What happens next

The tray fills and overflows.

On a wall-mounted split, the water runs down the inside of the wall. The first sign is a damp patch under the unit, or water dripping out of the face of the indoor unit onto the floor.

On a ducted or cassette system in a ceiling, the water goes straight into the ceiling cavity. That is far worse, because it spreads before it shows, and the stain that eventually appears is nowhere near the unit. It becomes a [ceiling water damage](/blog/water-coming-through-a-ceiling-in-an-apartment) job on top of the air conditioning repair.

Condensate pumps

Where gravity drainage is not possible, a small pump lifts the water.

Pumps fail. They are cheap, they run constantly in summer, and they sit in the same wet, dirty environment as everything else.

A pump with a failed float switch keeps running dry or stops running entirely. A pump with a blocked inlet cannot shift anything. Either way the tray overflows.

Most pumps have a safety switch that shuts the air conditioner down when the tray gets too high. Where it is wired in, it works well, and a unit that stops on a hot day is a far better outcome than a ruined ceiling. Where it was never connected, nothing stops the overflow.

Where the water is allowed to go

Condensate is wastewater. It goes to a proper drainage point.

What it must not do is discharge onto a neighbouring balcony, down a facade, or onto a path where it stains and grows algae. In strata buildings this is a recurring complaint, and it is usually an installation that took the shortest route rather than the correct one.

An outlet that drips onto the balcony below is the single most common air conditioning complaint in an apartment building.

The maintenance that prevents all of it

Clear and flush the drain line as part of the annual service, before summer rather than during it.

Clean the tray. Check the fall on the line and support any sag. Test the pump and its safety switch where one is fitted.

This is a small part of a [split system service](/blog/split-system-servicing-what-goes-wrong) and it prevents the most expensive failure the system can produce.

The early warning

Water dripping from the indoor unit, a musty smell when the system starts, or a gurgling noise from the drain.

Any of those means the line is partly blocked. It is a fifteen minute job at that point and a ceiling repair a month later.