🗓 Updated July 17, 2026 · EcoBack editorial team
Water under your portable air conditioner, or a tank-full light that keeps shutting it down mid-heatwave? Before you assume it's broken: on monobloc units, water is almost never a fault. It's condensate — the only question is why it's ending up on your floor. This guide covers portable monobloc units; fixed split systems leak for entirely different reasons.
It draws in warm room air, cools it and returns it — the waste heat leaves through the window hose. Real cooling, not just moving air around.
The one number every purchase hangs on — it is already worked out below. Adjust the room size if it is off.
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Narrow down the cause before buying — the spirit level costs least and settles most. Not lab-tested. Symbolic images.
Condensate drain hose
Drains the condensate continuously so the tank cannot overflow.
Small spirit level
If the unit stands at a slope the water leaves at the wrong place — check this before buying anything.
Condensate pump
Needed when the drain sits higher than the unit; without it nothing drains.
Why portable ACs collect water in the first place
When your unit cools, warm room air passes over an ice-cold evaporator coil inside. Moisture condenses on it — like droplets on a cold drink in summer. Depending on room humidity, that adds up to several litres of water per day. Every air conditioner does this; the difference is what happens to the water next.
Most modern portables use self-evaporation (often marketed as "auto-evaporation"): the condensate is slung onto the hot condenser coil, flashes into vapour and leaves through the exhaust hose. A small internal tank only catches the overflow. That's why many units run for weeks in dry heat without ever asking to be drained — and why visible dripping means something is interfering with that system.
Normal vs. not normal — the 30-second diagnosis
- Normal: the tank-full light coming on during muggy, thundery spells — in a really humid week, even daily; a few drops when you move the unit (water sloshing in the base pan); more water in dehumidifier mode.
- Not normal: a puddle spreading under a running unit; a steady drip from the housing; water from the air outlet; shutting off "full" minutes after you just emptied it.
In the second case, four culprits cover roughly nine out of ten leaks: a blocked or loose drain plug, a cracked base pan, a unit that isn't level, or an iced-up coil thawing all at once. Unplug the unit first, mop the floor dry so you can see where fresh water appears, then work through the checks below.
How to drain it properly (and find the leak on the way)
- 1. Check the drain plug. Low on the back sits at least one drain outlet with a rubber plug and screw cap. Not pushed fully home after the last emptying? That's the single most common cause. Press the plug in firmly, tighten the cap, check the rubber for cracks.
- 2. Empty the tank completely. Pull the bottom plug over a shallow tray (a bucket won't fit under the low outlet), tilting the unit slightly towards the drain until nothing more comes out.
- 3. Check the unit is level. A monobloc parked half on a rug edge lets condensate slosh past the pan and the sensor. Lay a spirit level on top in both directions — it must sit flat. This also matters for the window seal; see our guide to venting a portable AC through tilt-and-turn windows.
- 4. Clean the filter, and let an iced coil thaw. A clogged filter chokes the airflow, the evaporator gets too cold and ices over; when that ice melts, it dumps far more water than the pan can hold. Rinse the filter lukewarm every two weeks; if the fins behind it are white with frost, run fan-only for a few hours before cooling again.
Round-the-clock use? Fit a continuous drain hose
If you run the unit day and night through humid spells, connect a hose to the lower drain outlet and route it into a bucket, a floor drain or outside. Most monoblocs take a standard condensate hose (commonly 12–16 mm) costing a few euros. The one rule: it must fall continuously downhill — no upward loops — or water backs up into the machine. For dehumidifier mode, this setup is close to mandatory.
Why it shuts off when full — and why that's a feature
Inside the base pan sits a small float switch: when the water rises far enough, it cuts the compressor before the pan overflows. Annoying at 3 a.m., but it's the only thing between a humid night and a soaked floor. A unit that shuts off full immediately after draining, though, has a stuck float or a genuine fault — see below.
When a leak means service, not DIY
If you've worked through everything — unit dead level, plug seated, tank empty, filter clean, exhaust hose kink-free — and it still drips, you're left with genuine faults: a cracked condensate pan, a failed condensate pump, a stuck float switch, or low refrigerant making the coil ice up repeatedly. Refrigerant circuits and electrics are not DIY territory, and opening the housing voids a running warranty — contact the retailer or manufacturer instead. On an old unit out of warranty, a repair quote often exceeds its value; our round-up of the best portable air conditioners for a European heatwave covers current self-evaporating models, and how many BTU you need helps you size the replacement. Among our picks, the De'Longhi Pinguino PAC EX105 is consistently praised in public tests for its condensate handling in humid rooms.
Note: we haven't tested these units ourselves — we summarise public test results and user feedback.
Frequently asked questions
Why is my portable air conditioner leaking water?
Condensate itself is normal — cooling pulls moisture out of the air, and most modern units evaporate it out through the exhaust hose automatically. A visible leak usually means something simple: the tank is full, the rubber drain plug is loose or worn, the unit isn't standing level, or a dirty filter has let the cooling coil ice up. Work through those checks before assuming a fault.
How often should I drain a portable air conditioner?
It depends almost entirely on humidity. In dry heat, a self-evaporating unit can run for weeks without needing a drain. In muggy, thundery weather the same unit can fill its tank in a day or less — several litres of condensate per day is normal. Drain it whenever the tank-full light comes on; the float switch shuts the unit off safely if you forget.
Can I run a portable AC with a continuous drain hose?
Yes, if your unit has a lower drain outlet — most monobloc models do. Attach a condensate hose (commonly 12–16 mm) and run it with a constant downward slope into a bucket, floor drain or outside. Avoid any upward loops, or water backs up into the machine. For dehumidifier mode or round-the-clock use in humid weather, this is the most reliable setup.