Air Conditioning Condensate: The Drain Nobody Checks (and the Cause of 1 in 5 Water Damage Claims)
L'équipe Proclimo
22 Sep 2026 - 08 min read
One drip, then two. Then a brownish stain spreading across the wall, right below the indoor unit. In the best case, the owner slides a basin underneath and calls an installer. In the worst case, they're away on holiday, and it's the neighbour downstairs who comes home from work to find a ceiling soaked through.
Insurers know it well: since split systems started being installed everywhere — a trend accelerated by the 5.5% VAT rate on reversible air conditioning — water damage claims linked to an air conditioner have climbed year after year. And in the vast majority of cases, the cause is neither the compressor, nor the refrigerant, nor some spectacular installation blunder. It's the condensate drain: a 16 mm diameter hose that nobody ever looks at, right up until the day it overflows.
Where does all that water come from?
An air conditioner in cooling mode chills the air by passing it over a heat exchanger whose surface often drops to somewhere around 8 to 12 °C. The moisture-laden room air loses part of its water vapour, which condenses exactly as it does on a bottle taken out of the fridge. That water falls into a condensate drain pan located under the exchanger, then flows by gravity (or via a pump) outside or into a wastewater drain.
The volumes never fail to surprise:
| Situation | Condensate production (order of magnitude) |
|---|---|
| 2.5 kW split, flat, mild day | 0.3 to 0.8 L/h |
| 3.5 kW split, high humidity (coastal areas, South-West) | 1 to 2 L/h |
| 7 kW ducted system at peak heatwave | up to 3 L/h |
Over a heatwave night at 1.5 L/h, that's more than 12 litres of water that must leave the home without causing damage. The equivalent of a bucket, every single evening.
Beware of one common misconception: condensate isn't only a summer phenomenon. An air-to-air heat pump in heating mode produces water at the outdoor unit, during defrost cycles. A unit installed on a balcony with no drainage provision turns the slab into an ice rink as soon as the first December frosts arrive — a classic source of disputes between co-owners.
The five causes of overflow, in order of frequency
1. Biofilm in the pan and the hose
By far the number one cause. The condensate pan is damp, warm and enclosed: an ideal environment for bacterial and fungal biofilm. Season after season, this gelatinous deposit thickens, migrates into the drain hose and eventually forms a plug. Water then backs up into the pan, overflows the side of the unit and runs down the wall.
This biofilm isn't just a mechanical problem. ANSES, in its work on indoor air quality and air conditioning systems, points out that the damp parts of an installation can harbour micro-organisms whose dispersal into the air is undesirable. An indoor unit that smells of "wet rag" on start-up is almost always signalling a fouled pan.
2. Insufficient or reversed slope
Gravity drainage requires a continuous fall of at least 1 cm per metre (best-practice rules under DTU 65.16 on heat pump installations require drainage with "no counter-slope"). A hose that sags behind a piece of furniture, a wall penetration that rises slightly, an over-tightened cable tie crushing the tubing: water stagnates, biofilm establishes itself twice as fast, and the blockage arrives within one season instead of five.
3. A failed condensate pump
When gravity isn't an option (unit positioned lower than the drain, flat with no accessible façade), a condensate pump is fitted. It's a small float-operated device that draws water from the pan and lifts it up to 10 or 15 metres. An excellent solution… as long as it works. Its service life is around 5 to 8 years, and its float jams as soon as it gets dirty.
Crucial point: a properly wired pump has an alarm contact that shuts down the air conditioner when the water reaches a high level. Many low-cost installations skip this wiring. The result: the pump fails, the air conditioner keeps producing water, and the pan overflows all night long.
4. A poorly designed trap on a wastewater connection
Connecting condensate directly to the wastewater system is permitted, but requires an anti-odour, anti-backflow trap. Without a trap, sewer smells rise into the living room; with a trap that dries out over winter (no condensate for six months), the same thing happens. Ball-type or membrane traps avoid this nuisance.
5. Freezing of the hose at the wall outlet
In heating mode, defrost water runs at low temperature. A hose running three metres along a north-facing façade can freeze and form an ice plug. The water then backs up… inside the outdoor unit, and even as far as the exchanger.
Diagnosing it yourself before calling someone
Any work on the refrigerant circuit is reserved for holders of a certificate of competence for handling refrigerant fluids (F-Gas regulation and the French implementing orders). Condensate drainage, on the other hand, is plumbing: nothing prevents a homeowner from checking it, provided the power supply is switched off before any handling.
A few simple checks, to be carried out at the end of the cooling season — in other words, right now:
- Locate the outdoor end of the hose. Run the air conditioner in cooling mode at a low setpoint for 20 minutes on a humid day: water should drip steadily outside. Nothing coming out while the unit is running flat out? A blockage is likely.
- Inspect underneath the indoor unit. Limescale marks, drip trails, blistered paint below the appliance: the unit has already overflowed, even if you never saw it happen.
- Use your nose. A mouldy smell on start-up = fouled pan.
- Listen to the condensate pump. It should kick in for a few seconds at a time, then stop. A continuous hum or a completely silent pump both signal a problem.
- Check the routing. Follow the trunking: no low points, no crushed sections, no uphill runs.
For damp patches invisible to the naked eye — behind furniture, inside a stud partition — a pin-type or wave-type wall moisture meter will tell you in thirty seconds whether the plasterboard under the unit is sound or saturated. It's the accessory that saves you from discovering six months later that the glass wool insulation is soaking wet.
Maintenance: what you can do, what to delegate
The basic routine, twice a year
Cleaning the indoor unit's filters directly determines how clean the pan stays: less dust trapped in the exchanger means less organic matter to feed the biofilm. Rinse in lukewarm water every 4 to 6 weeks during heavy-use season, with filters completely dry before refitting.
For the exchanger itself (the fins visible when you lift the cover), an aerosol foam air conditioner cleaner, applied without dismantling anything, dissolves the deposits, which then drop into the pan and flush away with the condensate. The drain must absolutely be clear before you start, otherwise you'll turn a fouled pan into an overflowing one.
Clearing the hose
A lightly colonised hose can be treated with an anti-biofilm tablet for air conditioning dropped into the pan: it dissolves slowly and prevents the deposit from re-forming for several months. It's a consumable that refrigeration engineers fit as a matter of course during proper servicing; on a domestic split, a homeowner can do it themselves via the accessible pan.
A hose that's thoroughly blocked, on the other hand, needs blowing or sucking out — with a wet and dry vacuum cleaner connected to the outdoor end of the hose via a cloth acting as a seal, you'll sometimes retrieve a ten-centimetre plug of blackish gunk. A messy operation but an effective one, and with zero risk to the refrigerant circuit since you're only touching the water line.
What you never do yourself: open the refrigerant circuit, undo a flare fitting, "top up the gas". These operations are reserved for professionals holding a certificate of competence, and the company must hold a certificate of capability. Non-compliance is punishable, and your insurer will use it to refuse any claim.
The professional inspection
Under Decree no. 2020-912, air conditioning and heat pump installations with a nominal output of between 4 and 70 kW must undergo mandatory periodic servicing every two years, carried out by a professional. Condensate drainage is one of the points checked, alongside the condition of the circuit and its performance. Keep the servicing certificate: it's what will make the difference in the event of a dispute.
When the water reaches your neighbour: who pays?
This is where the subject stops being purely technical. A condensate overflow that soaks through a floor counts as water damage, handled under the usual rules of the IRSI convention for claims up to €5,000.
Three configurations, three lines of reasoning:
| Situation | Who covers it |
|---|---|
| Installation less than 10 years old, design fault in the drainage (counter-slope, missing trap, unsecured pump) | The installer's ten-year warranty — the work is unfit for its purpose |
| Older installation, unmaintained pan, no servicing certificate | The occupant's home insurance, with a risk of reduced payout for lack of maintenance |
| Tenant, air conditioning installed by the landlord | Routine maintenance (filters, accessible pan) falls to the tenant; replacing the pump or reworking the drain falls to the owner |
The decisive point, in every case, is proof. Dated photos of the pipe routing at the time of installation, an itemised invoice mentioning the condensate pump and its alarm, biennial servicing certificates: that file is what turns a "lack of maintenance" into an "original installation defect". Without it, the loss adjuster will almost always rule against the occupant.
One detail that costs dearly: many home insurance policies exclude damage caused by an appliance installed without regard for accepted trade practice. An air conditioner fitted by a DIY-minded friend, with no invoice and no certificate of capability, is an exclusion clause handed to the insurer on a plate.
The precautions to insist on at installation time
If your installation is still to come — and there are plenty this year — a few lines in the quote are worth more than ten years of litigation:
- Routing and slope of the drain described explicitly, with the wording "no counter-slope".
- A trap as standard whenever connecting to wastewater, preferably a membrane type.
- A condensate pump with an alarm contact wired to shut down the unit: insist on it in writing.
- An accessible hose, or at the very least one that can be dismantled along its lower section, to allow future unblocking without breaking open a partition.
- Insulation lagging on the sections exposed to frost and in unheated loft spaces (a cold hose in a warm loft condenses in turn and soaks the insulation).
- A drip tray or dedicated drain under the outdoor unit if it's installed on a balcony or terrace in a co-owned building.
For outdoor units on balconies, an anti-vibration mount for outdoor units with rubber feet solves two problems at once: it raises the appliance above the defrost puddle and reduces vibration transmitted into the slab — and therefore complaints from the neighbours.
The yearly calendar
- September–October: end of the cooling season, the ideal moment. Filters cleaned, pan checked, hose inspected before switching over to heating mode.
- November: check that the outdoor unit's defrost run-off isn't discharging onto a walkway.
- March–April: flush the circuit before the cooling season, replace the anti-biofilm tablet.
- Every two years: mandatory professional servicing (4 to 70 kW), certificate kept on file.
- Every 5 to 7 years: preventive replacement of the condensate pump, if the installation has one.
A well-installed air conditioner can run for fifteen years without any major incident. An air conditioner whose drain has never been looked at will, statistically, end up writing a registered letter on your behalf. Between the two, there are twenty minutes of checks, twice a year — and a file of evidence you'll be very glad to have the day the loss adjuster rings your doorbell.
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