Short answer: Air conditioning leaks water inside when the condensate it makes cannot drain away. The usual causes are a blocked drain pipe or tray, a failed condensate pump, a unit that is not level, or ice on the coil melting. Switch it off, catch the drips and, in a flat, warn the neighbour below before booking a repair.
Where the water comes from
When an air conditioner cools, moisture in the room air condenses on the cold indoor coil. On a humid London afternoon, a single unit can produce a lot of water this way. It runs into a drain tray under the coil and away through a small pipe, either by gravity to a gutter, waste pipe or soil stack, or with a small condensate pump that lifts it. If anything in that path fails, the water comes out of the front of the unit instead.
The common causes
| Cause | Typical sign | Fix |
|---|---|---|
| Blocked drain pipe or tray | steady drip from the front or bottom of the unit | clear and flush the drain |
| Failed or blocked condensate pump | dripping plus a pump that is silent or alarming | clean or replace the pump |
| Unit not level | water running out of one end | re level the unit or bracket |
| Ice on the coil melting | leaks after the unit has run a while, ice visible | fix the cause: dirty filters, low airflow or low refrigerant |
| Drain pipe with poor fall or a dip | dripping on hot days when water volume is high | reroute the pipe |
| Cracked tray or disconnected pipe | water inside the wall or behind the unit | repair or reconnect |
The first two are the most common. Dust and grime grow into a slime in the drain tray, and that slime blocks the outlet or the pump. Systems that have missed a service are the usual culprits.
What to do now
- Switch the unit off at the remote, then at its isolator switch if you know where it is.
- Catch the water with a towel and a bowl, and move anything electrical away.
- Warn the flat below if water could be getting into the floor or wall.
- Do not take the cover off or poke the drain.
- Note what you saw: where the water came from, whether you saw ice, and any error code.
- Book a check with an engineer.
Why it matters more in a mansion block
Wandsworth has a lot of mansion blocks, especially in Battersea along Prince of Wales Drive and the streets around Battersea Park and Clapham Junction, plus converted Victorian houses in Clapham, Balham and Tooting. In these buildings, a leak in your flat can soon become a stain on your downstairs neighbour’s ceiling. Many leases make the leaseholder responsible for damage caused by their own installations, and managing agents tend to ask for a condensate route that will not cause problems before they approve an install.
Older London buildings also make drain routing awkward. Solid walls, shared soil stacks and limited falls often mean the condensate has to be pumped, and pumps are the part most likely to fail. If your unit relies on a pump, an annual service that tests it is worth the visit. See our Wandsworth area page for more on local housing.
Ice and low refrigerant
If you see ice on the copper pipes or the indoor coil, the water is a symptom, not the cause. Ice forms when airflow across the coil drops, because of clogged filters or a failing fan, or when the system is low on refrigerant. Low refrigerant always means a leak somewhere, because a home split system is sealed. Topping it up without finding the leak just means it will leak again. Repairs to the refrigerant circuit must be done by an engineer with an F-gas qualification, working for a company certified by an approved body listed on gov.uk. Our guide to home air conditioning regassing explains the right approach.
Water from the outdoor unit
Water dripping from the outdoor unit is usually normal. In winter heating mode, the outdoor unit runs a defrost cycle that melts frost off its coil, and that water drains out of the base. Some installs pipe indoor condensate to drain near the outdoor unit as well. What matters is where it goes: it should not puddle on a path where it can freeze, or run down a neighbour’s wall.
Preventing it
- keep filters clean, so the coil does not ice up
- book a yearly service that cleans the drain tray and tests any pump
- do not block the drain outlet outside with plant pots or stored items
- if you are having a new system installed, ask how the condensate will drain and whether a pump is needed
Getting it fixed
Most leaks are simple to put right once the cause is found: clearing a drain, replacing a pump or re levelling a unit. If ice or low refrigerant is involved, the repair takes longer. Our air conditioning repair page explains how a diagnosis visit works.
Quick questions
Is it dangerous if my air conditioner is leaking water?
The water itself is condensate, not refrigerant, but it can damage walls, floors and ceilings below and can reach electrics. Switch the unit off and keep water away from sockets until it is checked.
Can I unblock my air conditioning drain myself?
It is safer not to. Drains run inside the unit and through walls, and poking or blowing through them can push blockages further or damage parts. An engineer can clear and test it.
Why is water coming from my outdoor unit?
That is usually normal. In heating mode the outdoor unit defrosts and drips water. In cooling mode, condensate may be piped to drain near the outdoor unit.
Last updated 23 October 2026. Written for homes in London. Rules can change, so check the linked sources for the latest position.