Air Conditioning Condensate: Where the Water Goes, and Why It's the Leading Cause of Leaks at Home

A brown stain on the ceiling, two metres below the indoor unit. A skirting board starting to warp. A cellar-like smell rising when the compressor kicks in. In the vast majority of cases, the appliance isn't broken: it's the water that isn't going where it should. An air conditioner doesn't just produce cold air, it also produces condensate — liquid water, in significant quantities, every day it runs. With the wave of installations triggered by the 5.5% VAT rate, insurers and refrigeration engineers are seeing the same thing: condensate-related claims have become the leading reason for call-outs outside of electronic failures. And almost every time, the blame lies with a few centimetres of badly fitted pipe.

Where this water comes from, and how much there really is

The principle is the same as a glass of iced water on a terrace. In cooling mode, the indoor unit's heat exchanger drops to a surface temperature of 5 to 12 °C. The ambient air, laden with moisture, passes over it, reaches its dew point and deposits its water vapour as droplets on the fins. This water trickles into a pan located beneath the exchanger, then flows away by gravity through a flexible pipe.

The orders of magnitude are often surprising:

In other words, an air conditioner installed in a sun-exposed living room can drain the equivalent of a pack of bottled water every week. That water has to leave the home, without backflow, without standing still, and without watering someone else's property.

A second, little-known point: in heating mode, the phenomenon is reversed. It's the outdoor unit that ices up, then defrosts, producing a puddle at the base of the wall. An owner who switches their air-to-air heat pump to heating mode in October sometimes discovers a sheet of ice on their terrace in December. We covered this in our checklist for switching over to heating mode: outdoor drainage is one of the points to check before winter.

The three possible set-ups — and which one causes trouble

1. Gravity drainage, the model pupil

This is the ideal arrangement. The condensate pipe runs downhill continuously from the pan to a discharge point. The professional rule, echoed in manufacturers' technical documents (Daikin, Mitsubishi Electric, Atlantic) as well as in the DTU 45.11 and 65.16 standards applied to drainage networks, is simple:

A minimum slope of 1 cm per metre (i.e. 1%), with no reverse slope, no low point, no unnecessary right-angle bends. In practice, serious installers aim for 2%.

The slightest sag in the flexible pipe creates a pocket of standing water. That pocket becomes colonised within weeks: biofilm, algae, dust — and the blockage is formed. The pan then overflows into the home.

2. The condensate pump: essential but noisy

When the discharge point is higher than the unit — the classic case of a floor-standing console, a ducted unit in a false ceiling, or a flat where drilling low down isn't possible — a condensate pump is added. A float detects the rising water and a small motor pushes the condensate towards a drainage network.

It's reliable, but it's one more mechanical component, and therefore one more potential point of failure. Three things to know:

• The pump must be fitted with a high-level safety switch that shuts down the air conditioner if it becomes saturated. Without it, the pump fails and the water goes into the ceiling. Check this point on the quote.
• It makes noise: 20 to 30 dB(A), a distinctive clicking sound. In a bedroom, that's enough to wake a light sleeper.
• Its reservoir gets clogged. It can be cleaned; it doesn't need replacing as a matter of course.

3. Discharge onto the façade or into a gutter: where disputes are born

The simplest connection for the installer is to drill through and let the pipe drip outside. That's legal on your own land, in a detached house, provided the water falls on your property. It becomes a problem as soon as it lands on a neighbour's land, on a balcony below, on a pavement or in a shared courtyard.

Article 640 of the French Civil Code prohibits worsening the water drainage easement to the detriment of the neighbouring property. In co-ownership buildings, condensate discharging onto the terrace below is a classic grievance at general meetings, and case law regularly rules against the co-owner who allowed a "shortest route" installation. There's also a health angle: this water, even when clear, is not drinkable and can carry micro-organisms from the exchanger's biofilm.

Connection to the wastewater network is also regulated: it requires a mandatory trap to prevent odours from backing up, and direct connection to a soil stack is prohibited under many departmental health regulations.

Seven signs that a condensate set-up is about to fail

You don't need to be a refrigeration engineer to spot shoddy work. Walk around the installation with a torch in hand, ideally a rechargeable head torch that leaves both hands free to follow the pipe behind a piece of furniture.

1. The pipe rises along part of its route before coming back down. Unacceptable.
2. It forms a soft "U" between two clips spaced too far apart. A condensate pipe should be supported roughly every 50 cm.
3. It is crushed where it passes through a drilled hole or pinched behind a trunking.
4. No trap is visible even though the connection is to a wastewater network.
5. The pipe dips into water in an inspection chamber or a container: guaranteed back-pressure.
6. The outdoor trunking isn't sealed at the wall penetration, or is drilled with a slight reverse slope towards the inside — rainwater gets in.
7. There's no drip tray beneath a ducted unit installed in a false ceiling above a living space.

That last point deserves particular emphasis. A ducted or cassette unit in a plenum above a living room, with no safety tray or detector, is a five-figure claim waiting to happen. Fitting a connected water leak detector at the low point costs less than €40 and sends an alert to your phone before the ceiling gives way.