Air conditioning condensate: where your AC's water goes and how to avoid damage
L'équipe Proclimo
25 Sep 2026 - 08 min read
Nobody thinks about it when signing an air conditioning quote, and yet: a 3.5 kW indoor unit can produce 1 to 3 litres of water a day in the height of summer, sometimes far more in a humid room. That water is condensate. As long as it drains properly, you will never see it. The day the circuit blocks up, you discover a stain on the ceiling, a cellar-like smell in the living room, or a furious neighbour because your AC is dripping onto their terrace. Here's how this invisible circuit works and how to keep it from becoming a problem.
Where does this water come from?
An air conditioner doesn't "make" cold: it extracts heat from indoor air by passing it over a very cold heat exchanger (the evaporator). Now, ambient air contains water vapour. On contact with a surface whose temperature drops below the dew point, that vapour condenses — exactly like the mist on a glass of iced water taken out of the fridge.
The amount of water produced depends on three parameters:
| Factor | Effect on condensate |
|---|---|
| Relative humidity of the room | The more humid the air, the heavier the condensation |
| Gap between setpoint and room temperature | A large gap makes the unit run longer |
| Running time | Proportional to the volume of water produced |
This is precisely why an air conditioner dehumidifies the air: the feeling of comfort doesn't come only from the drop in temperature, but also from this drop in humidity. The "Dry" (dehumidification) mode on your remote does nothing more than exploit this phenomenon while limiting the temperature drop.
Note that in heating mode, the phenomenon is reversed. It is the outdoor unit that condenses and frosts up. It then produces water on the ground during defrost cycles — a point often overlooked by owners of an air-to-air heat pump who discover a puddle, or even a sheet of ice, at the foot of their unit in December.
The water's journey, from tray to discharge point
On a standard wall-mounted split, the water always follows the same path:
- It runs down the fins of the evaporator.
- It falls into the condensate tray, a plastic gutter located beneath the heat exchanger.
- It flows into the drain pipe, usually a flexible 16 mm tube housed in the same conduit as the refrigerant lines.
- It reaches a discharge point: outdoors, a gutter, or the wastewater network via a trap.
Everything relies on one simple principle: gravity. The pipe must run downhill continuously, with a fall of at least 1 to 2 cm per metre. The slightest back-fall creates a pocket of stagnant water, and that pocket eventually becomes a plug of biofilm.
When gravity isn't enough: the condensate pump
In configurations where the discharge point is higher than the indoor unit — an AC installed on a garden level, in a basement, or a low-wall console beneath a window — the installer adds a condensate pump. This is a small box that detects the water level and pumps it upwards, sometimes over several metres.
Two things to watch out for:
- Noise. Some budget pumps switch on with an audible clicking, especially at night in a bedroom. Insist on a quiet model (under 21 dB(A)) and ask to hear it running before the job is signed off.
- Safety. A correctly wired pump shuts the air conditioner down in the event of a high-level fault. Without this alarm contact, the unit keeps producing water while the pump is dead: the tray overflows and the only warning you get is the stain on the ceiling.
warning
Always ask whether a condensate pump is included in the quote, and whether its alarm contact is wired in. It's a line item costing a few tens of euros that can prevent several thousand euros of damage.
Where are you allowed to discharge condensate?
This is the contentious question, in apartment blocks as much as in semi-detached houses. There is no single national text specifically covering air conditioning condensate, but several rules apply in cascade.
The departmental sanitary regulations (RSD) generally prohibit any water run-off onto public roads or onto a neighbour's property. A pipe dripping onto the pavement or onto the balcony below is a clear nuisance, and the town hall can take action.
The building's co-ownership rules and general meeting decisions govern the drilling of façades and connection to rainwater downpipes. Many co-ownerships allow air conditioning provided the condensate is connected to an existing drain and not discharged onto the façade.
The French Civil Code (article 640 et seq. on run-off easements, article 544 on property rights) allows an aggrieved neighbour to demand that the nuisance cease. Case law on the matter is consistent: an artificial run-off created by a piece of equipment benefits from no natural easement.
Connection to wastewater is possible in most municipalities, but it requires a trap to prevent odours coming back up, and sometimes an air-break disconnection. Conversely, some local sanitation regulations prohibit it, treating condensate as rainwater. Check with the sanitation department of your municipality or inter-municipal authority.
In practice, the order of preference followed by reputable installers is as follows:
- Connection to a rainwater downpipe or an existing inspection chamber.
- Connection to wastewater with a trap, if local regulations allow it.
- Discharge into open ground, on your own plot, with a bed of gravel to prevent erosion.
- Never: free discharge onto a façade, onto a shared terrace or onto a public road.
The five symptoms that reveal a condensate problem
1. Water dripping from the indoor unit
The absolute classic. In 8 cases out of 10, the drain pipe is blocked by a deposit of dust, algae and mould that forms a greyish jelly. The tray overflows and the water falls from the bottom of the unit's casing, often on the side opposite the drain.
Other possible causes: a back-fall in the pipe, a cracked tray, or an indoor unit mounted crooked (a fitting fault that shows up with a spirit level).
2. A musty smell on start-up
That typical "damp sock" smell comes from the biofilm that develops on the evaporator and in the tray, both permanently damp. It appears mainly when the unit is restarted after several days off.
This is a public health issue as much as a comfort one: ANSES regularly points out that poorly maintained air conditioning systems can be a source of microbiological contamination of indoor air. Cleaning the filters regularly and, once a year, the condensate tray, is part of the basic routine.
3. Weak airflow and grey filters
A clogged filter cuts the airflow, over-cools the heat exchanger and increases condensate production. The vicious circle often ends with frost on the evaporator, then a massive thaw that floods the tray. Simply cleaning the filters every two to four weeks during the season is enough to avoid it: a vacuum cleaner, lukewarm water, and a thorough drying before refitting.
4. A puddle under the outdoor unit in winter
Normal in heating mode — it's defrost water. It becomes a problem when it freezes on a walkway or seeps into a wall. The solution is to fit a drain tray under the outdoor unit connected to a drain, or to raise the unit on anti-vibration mounts with run-off towards a free-draining area.
5. Gurgling or a sewer smell coming back up
A sign of a badly sized, dried-out or non-existent trap on a wastewater connection. An air conditioning trap loses its water seal quickly, because the flow is low and intermittent. Membrane models (dry traps) solve the problem.
Maintaining the condensate circuit yourself: what you're allowed to do
Unlike the refrigerant circuit — strictly reserved for professionals holding an F-gas handling certificate — the water circuit contains no regulated fluid. You can therefore work on it yourself, provided you switch off the power supply before any handling.
The end-of-season routine, ideally carried out in September or October when switching over to heating mode:
- Switch off the circuit breaker dedicated to the air conditioning.
- Remove and clean the filters in lukewarm soapy water, and let them dry in the shade.
- Inspect the tray visible beneath the heat exchanger: look for deposits, stagnant water, black marks.
- Clean the pipe outlet outside: that's often where the blockage sits (insects, moss, leaves).
- Pour a little clean water into the tray and check that it comes out properly outside.
- Disinfect the evaporator with a suitable product.
For this last step, a no-rinse disinfectant cleaning spray for air conditioners, sold in aerosol form, lets you treat the heat exchanger and the tray in a few minutes. Avoid neat bleach, which is corrosive to the aluminium fins and to some condensate pumps.
If the pipe is blocked along its length, a manual vacuum pump for condensate (or, failing that, a simple low-pressure compressed air blower) will shift the blockage from the outside outlet. Always blow from the outside inwards, having first placed a container under the tray: the plug sometimes travels back the other way.
tip
A flexible brush for heat exchanger fins or a fin comb straightens the flattened blades of the evaporator and the outdoor unit. A heat exchanger with bent fins loses several percentage points of efficiency and condenses unevenly.
What you must not do on your own: open the refrigerant circuit, dismantle the control board, or remove the indoor unit from its wall bracket (risk of kinking a copper line). These jobs are for a refrigeration engineer, whose annual visit also remains mandatory for installations containing a refrigerant charge above the thresholds set by the F-Gas regulation.
Prevention rather than repair: good practice from installation onwards
Almost all condensate problems are decided at the time of fitting. A few points to check or insist on:
- A continuous fall on the pipe, with no loops or low points. Ask to see the route before the conduit is sealed up.
- A pipe of adequate diameter (16 mm internal minimum), and rigid or semi-rigid rather than a flexible corrugated tube, which traps deposits.
- Access to the discharge point, so you can clean it without acrobatics on a ladder.
- A drip tray under the unit if it is fitted above a plasterboard ceiling, wood flooring or electronic equipment.
- A connected water leak detector placed on the floor beneath the unit in sensitive setups: the device costs around twenty euros and sends an alert to your smartphone from the very first drops.
On the insurance side, water damage caused by condensate overflow is generally covered by the water damage guarantee of a comprehensive home insurance policy, provided the installation was carried out by a professional and has been maintained. This is exactly where the annual maintenance invoice proves its worth: in the event of a claim, it shows that you haven't left the installation to fend for itself. Without it, the insurer can argue a lack of maintenance and reduce the payout.
The special case of portable air conditioners
Portable monobloc units with a hose work on the same principle, but the water isn't drained away by gravity: it is either re-evaporated to the outside through the hose, or stored in a tank to be emptied by hand. In very humid weather, a 2-litre tank can fill up in half a day, and the unit shuts itself down as soon as it's full — usually at the worst possible moment.
Two solutions: choose a model with continuous drainage (a connector lets you fit a hose to a trap or a container), or put up with emptying it. For very humid homes, an electric air dehumidifier used alongside the AC eases the load on the air conditioner and noticeably reduces the amount of condensate the main unit produces.
In summary
| Situation | What to do |
|---|---|
| AC dripping indoors | Check for a blockage at the pipe outlet, switch the unit off |
| Musty smell | Clean the filters + disinfect the evaporator and the tray |
| Discharge onto façade or neighbour's property | Have the discharge point altered, risk of legal dispute |
| Noisy condensate pump | Insist on a quiet model, check the alarm contact |
| Puddle under the outdoor unit in winter | Normal (defrosting), plan for a tray or a drain |
Condensate is the most discreet part of an air conditioning system, and by far the leading cause of minor incidents… that cost a lot. A clean pipe, a correct fall, a compliant discharge point and an annual clean: four things to keep an eye on, one hour of work a year, and the assurance that you'll never wake up one August morning to a stain on the ceiling.
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