Boiler Leaking Water? What to Do and Why It Happens

You open the cupboard and the floor is wet, or there is a steady drip from somewhere under the boiler casing. Before you work out why, stop it getting worse: the fix can wait an hour, but the ceiling below and the wiring around the boiler cannot.

Short answer: Switch the boiler off at its fused spur or plug, close the cold water isolating valve under it, put a bowl or towel beneath the drip and keep water clear of any electrics. Then find where it is coming from. A drip from a valve or joint under the case is usually a quick repair. Water from inside the casing, or a pressure gauge that keeps climbing towards 3 bar, needs an engineer.

First two minutes: make it safe

1. Power off. Most boilers have a switched fused spur on the wall close by, sometimes labelled. Turn it off. If there is a plug, switch off at the socket only if the socket and your hands are dry.
2. Close the cold feed. Under a combi there is a row of pipes with small valves on them. The cold mains inlet usually has a valve you turn a quarter turn with a flat screwdriver, so the slot sits across the pipe. That stops fresh water feeding the leak.
3. Catch it. Bowl, baking tray or a folded towel underneath. If it is coming through the floor, put a bucket under the same spot downstairs.
4. Look, do not open. Leave the front cover where it is. Everything you need to work out the source can be seen from outside the case.

Where is the water coming from?

Dry the pipes and the bottom of the boiler with kitchen roll, then watch for a few minutes. The first place to turn wet is your answer.

A pipe joint or valve under the boiler

The most common leak, and the least worrying. Compression nuts and isolating valves loosen over years of heating and cooling, and the washers in older valves harden. You will see a bead forming on a nut or a valve body. Do not overtighten it yourself; a cracked olive turns a drip into a spray. This is usually a short repair.

The small copper pipe outside the house

Follow the pipework from the boiler to the outside wall. Somewhere near it a thin copper pipe pokes out, turned down towards the wall. If that pipe is dripping, or has a white limescale stain under it, the pressure relief valve is letting water out of the heating system. It does that when the pressure gets too high, often because the filling loop was left open or the expansion vessel inside the boiler has lost its charge. Our guide to a boiler losing pressure explains the gauge readings in detail.

The white plastic pipe (condensate)

Modern condensing boilers make a little acidic water as they run and drain it through a plastic pipe, usually to a drain outside or under the sink. If that pipe is blocked or frozen, water backs up and can spill from the boiler, and many boilers will also show a fault code and refuse to fire. In a cold snap, a frozen condensate pipe is the most likely reason. Pour warm water, never boiling, along the outside section until it runs, then reset the boiler.

Inside the casing

Water running from the bottom edge of the case, rust streaks on the casing, or a hiss from inside usually means a failed pump seal, a weeping automatic air vent or a corroded heat exchanger. None of these is something to tackle yourself. Leave it switched off and book a repair; the age of the boiler and the price of the part decide whether repair or replacement makes more sense, and we will tell you which before we start.

Drips only on very cold mornings

A few drops on the bottom of the case in winter, which dry up later, can be condensation from cold pipes in an unheated loft or garage, not a leak at all. Wipe it and check again when the heating has been on for an hour.

Keep an eye on the pressure gauge

The dial or digital display on the front tells you a lot. Note the reading with the boiler cold. If it falls day by day, water is escaping from the heating circuit somewhere, perhaps at a radiator valve rather than the boiler; our page on a leaking radiator covers those. If it rises close to 3 bar when the heating runs, the system is over-pressurised and the relief valve will keep opening.

When to call straight away

• Water is reaching a light fitting, a socket or the consumer unit.
• It is coming through a ceiling. Our page on water coming through the ceiling has the next steps.
• The leak is from inside the casing, or you can hear hissing.
• You have no heating or hot water and someone in the house is elderly, unwell or very young.

For anything else, a contained drip can wait until morning with the boiler off and a bowl underneath. When the boiler has also stopped working, our emergency boiler repair team covers London around the clock.