Boiler leaking water
Boiler leaks usually come from the pressure relief pipe, a joint on the pipes underneath, or a part inside the case. Where the drip starts tells you which.
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Boiler leaking water? Make it safe first
- Smell gas? Leave the leak. Call 0800 111 999, open doors and windows, and do not turn switches on or off.
- If the switch is dry and water is nowhere near it, switch the boiler off at the plug or fused spur. If water is near the controls or wiring, keep away, and switch off at the fuse box only if it is safe and dry where you stand.
- Put a tray or towels under the drip.
- Do not take the front cover off. Inside the case is work for a Gas Safe registered engineer.
Where a boiler leaks from
Dry the pipes under the boiler and the floor, and put dry kitchen roll under each joint. Only run the heating to watch for drips if the water is nowhere near the controls or wiring. Otherwise leave it off and note where the water was for the engineer.
| Where it drips | What it usually means | Who fixes it |
|---|---|---|
| 1From the small copper pipe that runs outside | The pressure relief valve is letting water out because the system pressure is too high, or the valve itself has failed. | Gas Safe engineer |
| 2From under the case | A seal, the pump or the heat exchanger inside the boiler is leaking. Rust marks on the case are a clue. | Gas Safe engineer |
| 3From a joint or valve on the pipes below | A compression joint or valve is weeping. Look for green or white crust on the fitting. | Heating engineer or plumber |
Check the boiler pressure gauge
Most combi and system boilers have a pressure gauge on the front. Your boiler's manual gives the right range, and many gauges mark it in green.
- In or near the red, and water from the outside pipe: the system is over pressure and the relief valve is doing its job. Common causes are an expansion vessel that has lost its charge, or a filling loop left open.
- Keeps dropping, and you keep topping it up: the system is losing water somewhere. Check the small copper pipe outside first. If it is dry, and so are the boiler and every radiator, the leak may be on a hidden pipe.
- Steady and normal: the leak is probably a joint or valve you can see, or the water is not coming from the heating at all.
Boiler makers and heating firms list the relief valve, worn seals, corroded fittings and the expansion vessel as the usual causes, and all advise a Gas Safe registered engineer for anything inside the case.
An overnight check
Turn the heating off in the evening and write down the gauge reading. Look again in the morning before the heating comes on. A drop with nothing dripping anywhere points to a leak you cannot see.
Who fixes a leaking boiler
By law, gas work must be done by someone on the Gas Safe Register. Replacing the relief valve or anything inside the case means opening the boiler, which is work for a Gas Safe registered engineer. A plumber or heating engineer can fix a leaking joint on the pipes underneath, and many are Gas Safe registered as well.
If you rent, tell your landlord straight away. Landlords must keep gas appliances safe and have them checked every year.
Water under the boiler that is not the boiler
Water under a boiler does not always start at the boiler. It can run down from a pipe above, come from a cold water pipe in the same cupboard, or appear because a hidden heating pipe is leaking and the boiler keeps refilling.
Dry the boiler completely. If the case and every joint stay dry while the puddle comes back, the water is arriving from somewhere else. The leak finder takes you through the next checks, and the radiator guide covers the rest of the heating.
Pressure keeps dropping and nothing drips?
If the boiler loses pressure while every joint, every radiator and the small copper pipe outside are dry, the heating may be leaking from a pipe under a floor or inside a wall. ADI traces hidden heating leaks and marks the leak on site before any repair is discussed.
- 25 leak investigation engineers, around 5,000 jobs a year across England, Scotland and Wales
- A fixed price quoted before you book, and a written report you can give your insurer
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ADI figures as of September 2026, from ADI's About page. Trustpilot rating read 28 September 2026.
Boiler leaking water: common questions
Is it safe to use a boiler that is leaking water?
If the water is near the controls, the wiring or the plug, no. Keep away from the wet electrics, switch off at the fuse box if it is safe and dry where you stand, and catch the water. A Gas Safe registered engineer should check the boiler before you use it again.
If you can smell gas, leave the leak and call the National Gas Emergency Service on 0800 111 999.
How much does it cost to fix a leaking boiler?
It depends on the part. A weeping joint under the boiler is usually a small job. A failed pump seal, expansion vessel or heat exchanger costs more, and on an old boiler an engineer may talk to you about replacement.
This site does not publish prices. Ask the engineer to tell you which part is leaking before you agree the work.
Is it normal for a boiler to leak water?
No. A boiler should not drip. Two pipes are designed to let water out. The pressure relief pipe releases water when the system pressure gets too high, so a drip there means something is wrong with the pressure.
A condensing boiler also has a plastic condensate pipe that carries waste water to a drain. Water from the end of that pipe is normal. Water leaking from a split in it is not.
Is a leaking boiler an emergency?
Treat it as urgent if water is near electrics, if it is more than a drip, or if you smell gas. A small, steady drip you can catch in a tray can usually wait for a booked engineer, but do not leave it for weeks: water inside the case damages parts and wiring.