Every air conditioning indoor unit makes water when it cools. Warm, humid air passes over the cold coil, moisture condenses on it, and the water runs into a tray and away through a drain pipe or pump. That’s normal. A leak means that water isn’t getting away as it should, or water is forming somewhere it shouldn’t.
Below are the usual causes, roughly in the order we find them, and what to do about each.
What to do right now
- Switch the system off at the remote or controller. That stops more condensate forming.
- Put a towel or bowl under the drip and move anything electrical or valuable out of the way.
- If water is near electrical fittings, a light or a socket, switch off the isolator for the unit and keep clear.
- Note any error code on the display. It helps us diagnose the fault.
- Check the filters (see below). If they’re clogged, that may be the cause.
Leave the refrigerant side alone. Anything inside the casing beyond the filters is a job for an engineer. Our repair service starts with a diagnosis call-out from £75 + VAT.
1. Blocked condensate drain
This is the most common cause. Dust that gets past the filter, slime and algae build up in the drain tray and pipe until water backs up and spills over the front or bottom of the unit. It often starts on the first humid days of summer, when the system makes more water than usual.
Fix: an engineer clears and flushes the tray and pipe, checks the outlet outside isn’t blocked by leaves or soil, and makes sure water runs away properly. Regular servicing prevents most of these.
2. Failed or blocked condensate pump
Where there’s no natural fall for a drain, a small pump lifts the water away. Pumps can block with sludge, their float or sensor can stick, or the pump itself can fail. A pump that’s gone quiet, or one that runs constantly, are both warning signs. Our guide to condensate pumps explains how they work.
Fix: clean the pump reservoir and sensor, check the discharge pipe isn’t kinked or blocked, or replace the pump.
3. Ice on the indoor coil
If the coil gets too cold it ices over. When the ice melts, often when the unit switches off or goes into a different mode, more water arrives than the tray can handle. Causes include blocked filters, a slow indoor fan, very low set temperatures for long periods, and low refrigerant from a leak.
Fix: clean the filters and let the unit thaw fully with the system off. If it ices again, call an engineer: a refrigerant leak needs finding and repairing, not just topping up. Our guide to ice on air conditioning units has more detail.
Want it done properly? AirCon Doctor installs, repairs and services air conditioning and heat pumps across London and the South East, from our base in East Grinstead. We’re an F-Gas certified company and NICEIC approved. Get a fixed-price quote.
4. Drain pipe with the wrong fall, or damaged
A gravity drain must fall continuously from the unit to the outlet. A pipe that sags, rises in the middle or has been knocked loose behind trunking will hold water and back up. Sometimes it’s an installation fault that only shows in humid weather; sometimes building work has disturbed the pipe.
Fix: re-route or re-fix the pipe with a proper fall, or add a pump where a fall isn’t possible.
5. Indoor unit not level
The drain tray only works if the unit sits level or slightly towards the drain side, as the manufacturer’s installation manual sets out. If the mounting plate was fitted out of level, or the wall has moved, water collects at the wrong end and drips.
Fix: re-level the unit on its mounting plate.
6. Sweating pipes or ducts
Sometimes the water isn’t from the unit at all. Cold refrigerant pipes, or ducts on a ducted system, will collect condensation on their surface if the insulation is damaged, missing or not sealed at joints. In a loft or ceiling void, that can show up as a damp patch some distance from the unit.
Fix: replace or seal the insulation along the full length of the pipe or duct.
Water from the outdoor unit
An outdoor unit dripping water is usually normal. In heating mode, the outdoor coil collects condensate and, in cold weather, frost that melts during defrost cycles. Plan where that water goes, especially above paths, so it doesn’t become a slip hazard in winter. Wall-mounted outdoor units can have a drain fitted to take it away.
Quick answers
Is it safe to keep using air conditioning that’s leaking water?
Switch it off until the cause is found. Water near electrics is a risk, and continued dripping can damage walls, ceilings and furniture.
Can I clear the condensate drain myself?
You can clean the filters. Clearing drains and pumps inside the unit is best left to an engineer, because it involves opening the casing near electrical parts.
Why does my air conditioning only leak on humid days?
More humidity means more condensate. A partly blocked drain or a pipe with a poor fall may cope on dry days but overflow when the system produces more water.
Related services
More advice
- Air Conditioning Condensate Pumps: When You Need One
- Ice on Your Air Conditioning Unit: What It Means and What to Do
- Air Conditioning Not Cooling: Causes and How to Fix It
- Air Conditioning Annual Service: What’s Included and Why It Matters
- All advice
This guide is general information, checked against the sources linked on 30 October 2026. It isn’t legal, tax or financial advice. Rules, grants and product ranges change, so check the current position before you rely on it.