Air Conditioning Condensate: Where the Water Really Goes and How to Avoid Damage
L'équipe Proclimo
1 Oct 2026 - 08 min read
The damage almost never shows up during the heatwave. It shows up three weeks later, when a stain appears beneath the living-room split unit, or when the neighbour downstairs reports a mark on their ceiling. The water, meanwhile, had been running since the start of summer — it was simply going into the wall instead of into the drain. An air conditioner or an air-to-air heat pump in cooling mode isn't just a thermal machine: it's also a dehumidifier that produces, for a 3.5 kW split unit in summer, between 1 and 4 litres of water per day depending on ambient humidity. That water has to go somewhere. Here's where, and everything that can go wrong.
Where this water comes from
In cooling mode, the indoor unit's heat exchanger typically drops to between 7 and 12 °C. Room air, loaded with water vapour, passes across these cold fins and crosses its dew point: the vapour condenses, exactly as it does on a glass of iced water sitting on a table in August.
The volume depends on two parameters and nothing else: the absolute humidity of the air being treated, and the air flow rate being circulated. In practical terms:
| Situation | Relative humidity | Estimated condensate / day |
|---|---|---|
| Dry flat, 28 °C indoors | 40 % | 0.5 to 1 L |
| Standard home during a heatwave | 55 % | 1.5 to 2.5 L |
| Mediterranean or Atlantic coast, night-time | 70-80 % | 3 to 5 L |
| Open-plan kitchen + laundry drying indoors | > 75 % | up to 6 L |
Multiply that by the number of indoor units on a multi-split system and you'll see why the condensate circuit is not an installation detail. On a 3-split system by the sea, you're looking at 10 to 15 litres a day running through pipes with a 16 mm internal diameter buried inside walls.
One point that is often misunderstood: in heating mode, it's the outdoor unit that produces the condensate, since it's the one that becomes cold. An air-to-air heat pump running a defrost cycle discharges water beneath the outdoor unit. Hence the sheets of ice on terraces in January, and the need for a properly drained defrost tray, or even a heating cable in cold climates.
The circuit, link by link
The collection tray
Beneath the indoor unit's heat exchanger, a plastic gutter collects the droplets. It's a shallow component, often crowded by the fan barrel and the motor, and it's where everything starts to go wrong: dust that slipped through a dirty pre-filter, spores, biofilm. Within a few seasons, a grey, slightly gelatinous sludge forms and migrates towards the drain outlet.
The drain pipe and its slope
The pipe (flexible or rigid PVC, 16 or 20 mm) must fall continuously, with a minimum slope of around 1 to 2 cm per metre. Without enough slope, the water stagnates; stagnant, it builds up biofilm; loaded with biofilm, it blocks. The absolute classic is the "belly": a section of pipe that rises and then falls again because the installer didn't fit clips every 50 cm. That low point becomes a permanent reservoir of water and a parasitic trap.
The trap
When the drain connects to a wastewater system, a trap is essential — otherwise sewer odours travel back up through the split unit's grille, and the drawn-in air can even break the flow. But a condensate trap poses a specific problem: out of season, the water evaporates, the water seal disappears, and odours get through. This is cause number one of "my AC smells bad when I switch it back on in June".
The discharge point
Three possible destinations:
- The home's wastewater system (sink, washbasin, stack): the clean solution, with a trap.
- Free discharge outside: simple, but effectively prohibited as soon as you're dripping onto a neighbour's balcony or onto a public right of way. Many leasehold/condominium regulations and municipal by-laws address it explicitly.
- Rainwater drainage: tolerated in some municipalities, banned in others. Check with your local authority's sanitation department, since condensate is treated as wastewater-equivalent under several collective sanitation regulations.
Air conditioning condensate is very low in minerals, slightly acidic, and potentially colonised by bacteria. It is not drinking water, nor is it water recommended for watering a vegetable garden.
The condensate pump: when gravity isn't enough
In an older flat, gravity drainage is often impossible: the split unit sits 2.20 m above the floor, the sink is 12 m away in another room, and there's no usable external wall. In that case you install a condensate lift pump.
Two families:
- Mini-format "peristaltic" or piston pumps, housed in a casing behind the unit or inside the trunking. Detection by float or capacitive sensor. They lift 5 to 10 m in height and deliver 10 to 15 L/h — more than enough.
- Tank pumps, fitted beneath the unit or in a suspended ceiling, with a reservoir of a few decilitres and a mechanical float. Noisier, but more tolerant of dirt.
The crucial point is the alarm contact. Any serious pump has a dry contact that, in the event of an abnormally high level, shuts down the machine (or at minimum the compressor). If that contact isn't wired to the indoor unit's board — and it isn't, in an uncomfortably high proportion of rushed installations — the pump fails, the water rises, and the only thing that warns you is the stain on the ceiling. Insisting that the alarm contact wiring appear on the quote is the best euro of prevention in the entire installation.
Pumps are also the most predictable wear part of the system: fouled check valve, float jammed by biofilm, tired diaphragm. Realistic service life: 5 to 8 years in seasonal use. Keeping a compatible condensate lift pump in reserve is hardly absurd if you live in a Mediterranean region where a call-out in the middle of a heatwave means a three-week wait.
The five faults that cause leaks
1. Tray blocked by biofilm. Typical symptom: water overflows from the right-hand side of the split unit, often dripping after 20 to 30 minutes of operation. It's the most frequent case and it is almost always the result of neglected filter maintenance.
2. A belly in the pipe. Water escapes intermittently, sometimes with a gurgling sound. This requires redoing the pipe fixings, not just clearing the blockage.
3. Trap dry or missing. No leak, but odours and sucking noises. Sometimes water backing up when the stack is under load.
4. Condensate pump failure. A sudden, heavy leak, typically on the first very hot day after a winter shutdown. Check the power supply, the float, and clean the check valve.
5. Frozen outdoor drain. In heating mode, at -5 °C, the external section of pipe blocks with ice. The water backs up, overflows into the defrost tray, and you end up with an ice rink under the outdoor unit. Solution: a decisive slope, a generous pipe diameter, and a self-regulating heating cable on the exposed section.
The maintenance you can do yourself
Cleaning the condensate circuit is among the tasks a careful homeowner can handle, provided you switch off the unit's power at the breaker before anything else. Nothing in this list touches the refrigerant circuit, which remains the preserve of professionals holding the relevant certification.
At the start of the season (May-June):
- Clean the pre-filters in warm soapy water and let them dry completely.
- Inspect the tray visually: if it's grey and slimy, action is needed.
- Pour half a litre of warm water into the tray and check that it comes out at the discharge point in under a minute. This is the simplest and most informative test there is.
- If nothing comes out: vacuum the drain outlet from the discharge end with a wet and dry vacuum cleaner, often enough to shift the blockage.
- Treat the tray with anti-algae tablets designed for condensate trays, which limit biofilm regrowth over the season.
During the season:
- Clean the filters every two to three weeks in intensive use.
- Once a month, look under the split unit to check for any dampness on the wall, however slight.
- Keep an eye (and an ear) on the condensate pump: a clicking cycle that gets longer signals a fouled check valve.
At the end of the season (October):
- Rinse and dry the filters before winter storage.
- Re-prime the trap with a little water if the unit is going to sit idle.
- For units installed in suspended ceilings or lofts, a connected water leak detector placed in the secondary tray costs a few tens of euros and warns you before the plaster gives way.
Condensate and air quality: the direct link
A stagnant tray is a culture medium. ANSES, in its work on indoor air quality, and INRS, for air conditioning systems in the workplace, have long identified standing water in air-handling systems as potential reservoirs of bacteria and mould. On a domestic split unit, the legionellosis risk remains very low — there's no water aerosolisation as there is in a cooling tower — but the risk of spreading mould spores is real and well documented by the characteristic "damp earth" smell that many users recognise.
Three habits that make a difference:
- Let the heat exchanger drying function run (often called Dry, Clean or Self-clean depending on the brand) at the end of use: the fan runs for 10 to 30 minutes without cooling and dries out the fins. It's free and highly effective.
- Never spray bleach into the tray: it attacks plastics and copper, and does nothing about the blockage. A foaming cleaner formulated specifically for air conditioner heat exchangers is better suited.
- In damp homes, measure rather than guess: a indoor thermo-hygrometer costing €15 will tell you immediately whether your problem lies with the AC settings, the ventilation system, or water ingress.
Who pays in the event of water damage
This is the awkward question, and it hinges on the cause.
- Installation defect (no slope, missing trap, pump not wired to its alarm): the installer is liable. If the installation is less than ten years old and affects the dwelling, the ten-year structural warranty may apply; short of that, the completion warranty and professional liability insurance. Keep the quote, the invoice and the company's insurance certificate.
- Lack of maintenance (filters never cleaned, tray blocked for three years): your home insurance will generally cover the damage under water damage cover, but the insurer may reduce the payout in cases of clear negligence, and the deductible is yours to bear. Keep your maintenance records.
- Damage to the neighbour downstairs: declare it within five working days, complete a joint water damage report between the two insured parties, and in a shared building inform the managing agent. If the leak originates in a private part of the property, your insurer takes the lead.
One simple, inexpensive tip: photograph the installation when the work is finished — pipe slope, trap, pump and its wiring — and file those photos with the invoice. On the day the damage happens, that file is worth far more than a trail of emails.
What to insist on in a quote
Before signing, four lines must appear in black and white:
- The route and method of condensate drainage (gravity or pumped, with the exact discharge point).
- The presence of a trap if connecting to wastewater.
- The wiring of the condensate pump's alarm contact to the indoor unit.
- Drainage of the outdoor unit's defrost tray, for a reversible model used for heating.
These four points cost almost nothing to install and account for the bulk of the water damage risk over the equipment's service life. An installer who spells them out unprompted tells you more about their professionalism than any sales pitch about SEER ratings.
Water, in an air conditioning system, is the only fluid you can see and touch. It's also the one that causes the most material damage. Two minutes of checking a month during the season is enough to make the problem invisible — in the good sense of the word.
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