The short answer
Most water leaks are a drainage problem, not a refrigerant one. Air-con units produce condensation by design, which should drain away. A leak usually means a blocked or sloped-wrong condensate drain, a frozen coil that melts faster than it can drain, a failed condensate pump, or a poor installation. You can clear a simple blockage and check the filter; persistent leaks, icing or refrigerant issues need an F-Gas-certified engineer. Untreated leaks cause damp and mould.
Water under an indoor air-con unit alarms people, but it is rarely a refrigerant leak — it is almost always condensation that has failed to drain away. This guide explains why your unit makes water at all, the common reasons it ends up on your floor instead of down the drain, what you can safely sort yourself, and when to call an engineer before the damp does any damage.
Water leaks at a glance
- Most common cause Blocked condensate drain
- It’s usually Condensation, not refrigerant
- You can fix Clear simple blockages, check filter
- Needs an engineer Icing, pump faults, poor install
- If ignored Damp, staining, mould
- Refrigerant work Certified engineers only
Why air-con makes water in the first place
An air-conditioner cools by passing warm, humid room air over a cold coil. Just as a cold drink sweats on a summer day, moisture condenses out of that air onto the coil — this is completely normal and is actually a useful side-effect, gently dehumidifying the room. That condensate collects in a tray beneath the coil and drains away, either by gravity through a pipe or, where the unit sits below the drain point, via a small condensate pump. A leak happens when that water cannot get away cleanly. So the first thing to understand is that water itself is normal; water on your floor is a drainage fault.
The amount of water a system makes depends on the humidity of the air it is treating, which is why leaks often show up on muggy days when the unit is condensing fast. On a humid afternoon a hard-working indoor unit can produce a surprising volume of water, all of which has to find its way out through one small drain. That is exactly why the drainage path needs to be clear and correctly fallen: when it works, you never notice it; when it clogs even slightly, the tray overflows and you find water where it should not be.
The common causes
Most indoor-unit leaks trace back to one of a handful of problems.
- Blocked condensate drain — algae, dust or sludge clogs the pipe so water backs up and overflows the tray.
- Frozen coil — a low charge or restricted airflow ices the coil; when it melts, water arrives faster than the drain can cope.
- Failed condensate pump — on units that pump water uphill, a stuck or dead pump means the tray overflows.
- Poor installation — a drain run that is too flat or back-falling, or a unit not mounted level.
- Dirty filter — restricting airflow, which can ice the coil and cause overflow.
| Symptom | Likely cause | Who fixes it |
|---|---|---|
| Steady drip from indoor unit | Blocked condensate drain | You (simple) / engineer |
| Ice then water when it melts | Frozen coil — low charge/airflow | Engineer |
| Water with the unit below drain | Failed condensate pump | Engineer |
| Leak from day one | Installation fault | Installer |
What you can safely do
Some leaks have a simple fix. Switch the unit off to stop more water forming, then check and clean the filter — a clogged filter is a frequent root cause. Look at the visible condensate drain outlet outside: if it is blocked with debris, gently clearing it can restore flow. Mop up promptly to avoid damp and staining. If you can see ice on the coil or pipes, switch the unit off and let it fully defrost before doing anything else — running it iced up makes the leak worse and can damage the compressor. What you should not do is open the refrigerant circuit or attempt electrical work on the pump.
When icing points to refrigerant
If the leak is caused by a frozen coil, the underlying problem is usually restricted airflow or a low refrigerant charge — and a low charge means a leak in the sealed circuit, which is illegal to touch unless you are F-Gas-certified. The ice itself is a symptom: as it builds and then melts, it overwhelms the drain and water spills out, so chasing the leak without addressing the icing solves nothing. In that case the right route is a certified engineer who can diagnose the icing, find any refrigerant leak and recharge correctly — see re-gas explained and air con not cooling. Keeping up with an annual service, which includes flushing the drain, checking the charge and confirming the unit sits level, prevents the great majority of water leaks before they start — see servicing explained. This page is general guidance, not a diagnosis of your specific unit.
Air-con still leaking after clearing the drain?
Get matched with an F-Gas-certified engineer to find the real cause — drain, pump or refrigerant — and fix it before damp sets in.
Frequently asked questions
Is air-con water normal?
Producing water is normal — condensation forms on the cold coil and should drain away. Water leaking onto the floor is not normal and points to a drainage fault.
Why is my air-con dripping water inside?
Most often a blocked condensate drain, a frozen coil that melts faster than it drains, a failed condensate pump, or a poor installation. Check the filter and drain first.
Can I fix an air-con water leak myself?
You can clean the filter and clear a simple drain blockage. Icing, pump faults and anything involving refrigerant need an F-Gas-certified engineer.
Is a leaking air-con dangerous?
The water itself is condensation, but if left it causes damp, staining and mould. A leak from a frozen coil can also signal a refrigerant problem that needs an engineer.
Sources & further reading
- HSE — damp, mould and indoor environment guidance
- GOV.UK — F-gas: guidance for users, producers and traders
- Energy Saving Trust — cooling and dehumidification
- Manufacturer technical data (Daikin, Mitsubishi Electric) — condensate drainage
This guide is general information, not a site-specific survey or a substitute for a quote from an F-Gas-certified installer. Installation, servicing and refrigerant handling are legally restricted to F-Gas-certified engineers.