Air conditioning condensate drainage: summer's No. 1 breakdown and how to avoid it

"There's water running out of the split unit, but the refrigerant can't be leaking, can it?" No, and that's good news: in the vast majority of cases, the water seeping from an indoor unit has nothing to do with the refrigerant. It's condensation — pure water, pulled out of the air in your living room. The problem is that a 3.5 kW wall-mounted split produces between 0.8 and 2 litres per hour on a humid day, and that water has to go somewhere. When its path gets blocked, it finds its own: the wall, the floorboards, the suspended ceiling, or the flat below.

After the wave of installations triggered by the 5.5% VAT rate, refrigeration engineers all say the same thing: condensate drainage has become the leading reason for summer service calls, well ahead of low refrigerant charge. And it is also, by far, the most preventable fault of all.

Where exactly does this water come from

The principle is that of a cold bottle taken out of the fridge. In cooling mode, the indoor unit's coil drops to around 5 to 8 °C. The moisture-laden room air passes through it: the water vapour condenses on contact with the fins, runs down, and falls into a condensate tray located beneath the coil. From there, a flexible PVC pipe carries it away by gravity to the outside, or into a rainwater drainage system.

The amount produced depends directly on the humidity of the air, not on the appliance's output:

Indoor conditions	Condensate produced (3.5 kW split)
26 °C / 40% RH — dry air, south-east	0.3 – 0.6 L/h
27 °C / 55% RH — typical situation	0.8 – 1.2 L/h
28 °C / 70% RH — coastal, thunderstorm	1.5 – 2.2 L/h

Over a heatwave day with 10 hours of operation in a humid region, that means 15 to 20 litres of water to drain away from a single unit. A three-bedroom flat fitted with four splits can top 50 litres a day. No installation will tolerate improvisation on this point for long.

Two useful clarifications:

• In heating mode, the phenomenon is reversed: it is the outdoor unit that condenses, then frosts up. The water then falls beneath the outdoor unit, and that's where those patches of ice on balconies come from in winter — a subject in its own right.
• Condensate is demineralised water, but not drinkable: it has passed over a coil potentially colonised by biofilms. Don't use it for an aquarium or for cooking.

The five causes of failure, in order of frequency

1. Pipe blocked by biofilm

This is the great classic, responsible on its own for roughly half of all call-outs. The tray and the first section of pipe stay permanently damp throughout the summer. Dust, pollen, spores and bacteria form a brownish jelly — biofilm — that eventually clogs the internal diameter (often just 16 mm).

The typical symptom: the AC works perfectly in June, then starts dripping in late July, a little at first, then in earnest. Water overflows the back of the tray and runs down the wall.

2. Insufficient fall, or a reverse gradient

Gravity drainage requires a continuous fall of at least 1 cm per metre, with no low points. On a sloppy installation, the pipe sags behind a piece of furniture or rises slightly to get past an obstacle. Water stagnates in this improvised trap, biofilm establishes itself twice as fast, and the tray overflows as soon as the flow rate increases.

A conscientious installer checks the fall with a 60 cm spirit level at the time of installation; it's a thirty-second check that avoids a dispute worth several hundred euros.

3. Condensate pump failure

When the indoor unit sits below the discharge point — a common case in flats, in basements, or with a low-level console — gravity isn't enough. A condensate pump for air conditioning is then fitted, a small unit that draws water from the tray and pushes it up to 10 or 15 metres in height.

These pumps have a service life of 5 to 8 years and fail for three reasons: a fouled float, a burnt-out motor, or a punctured discharge pipe. On most models, a safety contact shuts down the AC when the level rises too high: your air conditioner switching itself off with no refrigerant error message is very often a flooded condensate pump, not a compressor problem.

4. Clogged filter and coil icing

A saturated filter reduces airflow. The coil, with less air passing over it, drops below 0 °C and ices up. When the unit stops, that ice melts all at once — several hundred millilitres in a few minutes — and the tray, sized for a steady trickle, overflows.

Here the water leak is only a symptom: the real problem is fouling, which also pushes consumption up by 10 to 25%. Monthly filter cleaning during the season is enough to prevent it, and a vacuum cleaner with a fine dusting brush does the job in two minutes per unit.

5. Low refrigerant charge

Rarer, but real: an insufficient charge causes the evaporating pressure to drop, the coil partially ices up, and we're back to the previous scenario. This case necessarily calls for a professional holding the certificate of competence for handling fluorinated refrigerants, working for a company holding the corresponding company certificate, in accordance with European F-Gas Regulation 2024/2215. No DIY possible.

Simple rule of thumb: if the leak appears gradually over the course of the summer, think blockage or fall. If it appears suddenly after a shutdown, think icing — so filter or refrigerant.

What the regulations say about discharging condensate

This is the point almost nobody checks before installation, and the one that poisons a great many apartment blocks.

Discharging onto public land is prohibited. Letting a pipe drip onto a pavement, a roadway or the street-facing façade of a building can earn you a formal notice from the local council. Several French departmental sanitary regulations (RSD), based on the model RSD, explicitly prohibit it as water discharge onto the public highway.

Discharging onto a neighbour's property or a common area is just as prohibited. In a co-owned building, a pipe dumping its 15 daily litres onto the balcony below constitutes an unreasonable nuisance to neighbours, with the added risk of damaging the waterproofing of the surface. The managing agent is entitled to demand that the installation be put right.

Connecting to the wastewater system isn't a free-for-all either. Technically possible, it requires a trap to prevent odours coming back up, and it is subject to local drainage regulations: some authorities prohibit discharging "clean" water (rainwater, condensate) into the foul sewer, precisely because it needlessly overloads treatment plants. Check with your local authority's drainage department before drilling anything.

The generally accepted solutions, in order of preference:

1. Discharge into the building's or house's rainwater system (downpipe, inspection chamber).
2. Soakaway into open ground on your own property, away from foundations and boundary lines.
3. Discharge to wastewater with a trap, if local regulations allow it.
4. Collection in a buffer tank and reuse for watering ornamental plants — bear in mind, non-potable water.

For a detached house, directing condensate towards a bed of plants rather than towards the foot of the wall is both free and more sustainable; a simple length of porous soaker hose extending the drain spreads the volume without carving out a gully.