Air conditioning condensate: where the water goes, what the law says and how to avoid damage

Nobody asks where the water goes. People discuss output, SEER ratings, the price on the quote, where the outdoor unit will sit — and then they sign. Then, one July morning, a dark stain appears on the living room ceiling, or the neighbour downstairs discovers that their balcony is permanently damp. An air conditioner or an air-to-air heat pump in cooling mode doesn't just cool the air: it dehumidifies it. And that humidity turns into liquid water, in quantities most owners drastically underestimate. Here's what becomes of that water, what the regulations require, and the setups that stop a summer comfort upgrade from turning into an insurance claim.

Where does this water come from, and how much of it is there?

The principle is the same as a bottle taken out of the fridge that becomes covered in droplets. Inside an indoor unit in cooling mode, room air passes over a heat exchanger whose surface temperature drops below the dew point. The water vapour in the air then condenses on the fins, runs down, and is collected by a tray positioned beneath the exchanger.

The actual volumes often come as a surprise:

The more humid the air, the higher the production. On the Atlantic or Mediterranean coast in August, a ducted system can produce the equivalent of a bucket of water a day. That water has to be drained away continuously, either by gravity or by a pump.

Watch out for a common misconception: in heating mode, it is the outdoor unit that produces the condensate, not the indoor one. The outdoor exchanger drops below 0 °C, frosts up, then defrosts — releasing a far from negligible volume of water at the base of the unit with every cycle. Many installations fitted for summer use only reveal the problem during the first winter of heating: a puddle in front of the patio door, a sheet of ice on a terrace, water running into a cellar.

What the regulations say: no, you don't let it run onto the pavement

Contrary to a persistent belief, condensate drainage is not a free-for-all. Several sets of rules apply, and together they leave little room for manoeuvre.

Discharge onto public land

Departmental sanitary regulations and, above all, municipal highway regulations prohibit, in virtually every French municipality, any discharge of water from private property onto public land: pavement, roadway, passageway. The model departmental sanitary regulation, still widely adopted by prefectures, sets out this general principle banning discharge onto public thoroughfares.

In practical terms: a condensate pipe dripping over a pavement is in breach of the rules, and the risk is not merely theoretical. In winter, defrost water freezing on a pedestrian crossing point exposes the owner to civil liability if someone falls. This is one of the few areas where the town hall acts quickly once a complaint is filed.

Connecting to the waste water system

Connecting the condensate pipe to an existing waste water outlet (a basin trap, a bathroom stack, a washing machine outlet) is possible, but never directly. Two rules, derived from the public health code and plumbing standards, apply:

• there must be a water seal (trap) between the condensate circuit and the waste water system, otherwise sewer odours will travel back into the home through the indoor unit;
• any risk of water backing up towards the appliance if the stack becomes blocked must be eliminated.

That is precisely why manufacturers require the fitting of a dedicated condensate trap, either membrane or ball type, usually ultra-slim so it can slide in behind the unit. A pipe pushed straight into a waste outlet, with no trap, is the number one cause of "air conditioners that smell bad" — and the diagnosis is immediate: the smell is that of the sewer, not of the machine.

Rainwater drainage

Connecting to a rainwater downpipe is the simplest solution and the one most often accepted, provided the network is genuinely separate and the municipality does not prohibit it. Beware of municipalities with combined sewer systems, and of apartment buildings where rainwater downpipes are common property: drilling into a shared downpipe requires authorisation.

Remember: condensate starts out as clean water, but it very quickly picks up dust, bacteria and biofilm in the tray. It is not drinkable and is not considered reusable rainwater without treatment.

Apartment buildings: the biggest source of disputes

In multi-occupancy buildings, condensate is as frequent a source of conflict as outdoor unit noise, and disputes are often settled faster — and against the installer.

The problem configurations:

1. The pipe that drips onto the balcony below. This is a clear-cut case of unreasonable nuisance to neighbours, and case law on this point is consistent: the appliance owner must rework the installation at their own expense.
2. Direct discharge down the façade. The water leaves limescale streaks and dirty marks on render or stonework. The managing agent can demand restoration of the façade, which is common property.
3. Drilling into a shared downpipe without approval from the owners' general meeting: the building's owners can demand that the connection be removed.
4. Discharge into an undersized gutter or roof channel, which overflows and causes water ingress.

Before any installation in an apartment building, the question "where does the condensate go?" must appear in black and white on the quote, and in the authorisation request submitted to the managing agent. It is a point that general meetings now examine as a matter of course, just like the visual impact of the outdoor unit.

Correct setups, from the simplest to the most technical

Gravity drainage

This is the benchmark solution: the pipe runs downhill with a continuous fall, with no counter-slope or low point, and a minimum gradient of 1 to 2 cm per metre. The indoor unit's tray must always sit higher than the discharge point.

The classic mistakes:

• a pipe that rises slightly and then falls again, creating a pocket of standing water → a biofilm blockage guaranteed within two seasons;
• a pipe crushed inside the trunking or pinched behind the unit;
• an excessive horizontal run with no supports, which eventually sags.

Checking this yourself on an existing installation is easy: a magnetic spirit level held along the trunking lets you confirm that the fall is downwards along the whole route. It's a two-minute check that explains half of all indoor unit leaks.