Air conditioning condensate: where the water goes, and why it sometimes ends up on your wall

"There's a water stain under the living-room air con, and it smells musty when it starts up." Repair engineers receive this message by the dozen every late summer. In the vast majority of cases it is neither a refrigerant leak nor an electronic fault: it is simply condensation water that no longer goes where it should.

We don't always realise it, but an air conditioner is also a dehumidifier. A 3.5 kW wall-mounted split running for ten hours in a humid room can extract 10 to 20 litres of water a day. That water has to leave the home via a dedicated, discreet circuit, often several metres long — and sometimes dependent on a small electric pump whose very existence the owner knows nothing about. This article explains where that water goes, why the circuit blocks up, and what to insist on at installation so you never have to file an insurance claim.

Where the water comes from: a little physics, no equations

The ambient air contains water vapour. When it passes through the heat exchanger of the indoor unit, whose fins are held at around 6 to 12 °C in cooling mode, it drops below its dew point: the vapour condenses into droplets on the fins, exactly as it does on a bottle taken out of the fridge.

These droplets trickle down into a condensate tray placed beneath the exchanger, then flow outside through a flexible pipe integrated into the refrigerant line set.

The volume produced depends directly on the humidity of the air:

Indoor conditions	Condensate produced (3.5 kW split, 8 hrs running)
26 °C / 45% RH (dry air, inland)	3 to 6 litres
28 °C / 60% RH (typical summer situation)	8 to 14 litres
30 °C / 75% RH (coastal, bathroom, kitchen)	15 to 25 litres

Two practical consequences. First, an air conditioner that produces no water at all in the middle of summer isn't being "efficient": there is a strong chance it is undercharged with refrigerant, or that its tray is overflowing somewhere else. Second, a unit in heating mode also produces condensate, but this time on the outdoor side: it is the outdoor unit that frosts up and defrosts, and the defrost water runs out beneath the appliance. That's the famous puddle — sometimes a sheet of ice — that appears on the terrace in January.

The three ways to drain it: gravity, condensate pump, external discharge

Gravity drainage, the option to favour

This is the most reliable, because it has no moving parts. The pipe runs downhill in a continuous fall from the tray to the discharge point, with a minimum slope of roughly 1 to 2 cm per metre (i.e. 1 to 2%). No counter-slope, no "belly" where water sits, no upward run.

That's why the position of the indoor unit is never a purely aesthetic choice: you need a downward path to the outside or to a wastewater drain. A good installer will tell you this at the survey stage. A bad one will put the unit "wherever you like" and try to compensate afterwards.

The condensate pump, when gravity isn't enough

When the unit is in a basement, behind a suspended ceiling or too low relative to the discharge point, a condensate lift pump is added: a small unit that detects the water level with a float and pumps the condensate up to 5 or 10 metres in height depending on the model.

It is a perfectly valid solution, but it introduces three weaknesses:

• a wearing part (motor, float, non-return valve) with a service life of around 5 to 8 years;
• operating noise, 20 to 35 dB(A), audible at night in a bedroom;
• dependence on electricity: a power cut, a blown fuse, and the tray overflows.

Decent models include a high-level safety: a second contact that shuts the air conditioning down if the level rises abnormally. Insist on it — it costs far less than redoing a ceiling. If your current installation doesn't have one, a connected water leak detector placed under the unit acts as a cheap safeguard — it triggers a smartphone alert at the very first drops.

Discharge onto the façade: legal, but unloved

Many installations simply discharge condensate drop by drop onto the façade or at the foot of the wall. Technically, no national legislation forbids this. But three situations cause problems:

• In a block of flats, the building rules may prohibit any run-off onto common areas or onto the balcony of the neighbour below. Continuous run-off onto a metal railing eventually stains and corrodes it.
• On the public highway, several French departmental sanitary regulations (RSD) include a ban on discharging water onto the pavement. Local councils rely on these to issue fines if someone complains, especially where the water creates a slippery patch.
• On a stone or light-coloured rendered façade, permanent damp causes blackish streaks and algae growth within a few seasons.

Best practice is to connect the condensate to a rainwater downpipe, or failing that to direct it to a soakaway bed at the foot of the wall. Connecting directly to the wastewater system is possible, but only with a trap: without one, sewer odours come back up into the home as soon as the appliance stops.

Why it blocks up (and why always in August)

The condensate tray is an ideal environment for micro-organisms: lukewarm water, darkness, organic dust. A gelatinous biomass forms there — "biofilm" — which migrates into the pipe and eventually forms a blockage.

Three factors speed the process up:

1. Clogged filters. The dust that gets past the exchanger sticks to the damp fins and then falls into the tray. Cleaning the filters every 2 to 4 weeks during the season is the single most cost-effective maintenance task there is. A powerful handheld vacuum and warm soapy water are more than enough.
2. A unit installed in a kitchen. Airborne grease settles on the exchanger and turns the biofilm into a sticky paste that is virtually impossible to shift without dismantling.
3. A long winter shutdown. The residue dries, hardens, and the blockage forms at the first restart. Hence the peak in call-outs in late June and in August.

When the pipe blocks, the tray overflows. On a wall-mounted split, the water comes out of the side of the casing or drips through the discharge grille, and the stain appears on the wall. On a ducted system or a ceiling cassette, it goes through the suspended ceiling — and that's when the bills start getting serious.