Boiler leaking water? Here’s what it usually means
Guidance last checked on against UK safety regulation and the sources behind our service pages.
Where the water appears names the fault. From the outside pressure-relief pipe: an over-pressurised system or a failed expansion vessel. From under the casing: a pump seal, a corroded heat exchanger, or a failed auto air vent. From a joint behind the boiler: pipework, not the boiler. Turn the boiler off, catch the water, and keep it away from electrics.
Boiler leaking water: the questions people ask
Why is water dripping from the pipe outside?
Water dripping from the pressure-relief pipe — the one that runs from the boiler through an outside wall — means the system pressure is too high for the safety valve to hold, or the internal expansion vessel that absorbs normal pressure changes has failed. An engineer can usually tell which from the pressure gauge's behaviour after a top-up: a fast climb back to the same fault points at the vessel, not just an over-fill.
Why is water pooling under the boiler's casing?
A leak that only shows up as water pooling or staining underneath the boiler's own casing, rather than from a visible external pipe, points to something failing inside: a worn pump seal, a corroded heat exchanger, or a failed automatic air vent letting water weep out. None of these are safe to open up yourself on a gas boiler: it is gas work, which by law needs someone competent, and any engineer you pay must be Gas Safe registered.
Why is water coming from a pipe joint behind the boiler?
Water appearing at a joint in the pipework behind or beside the boiler, rather than from the boiler's own casing or its pressure-relief pipe, is very often the pipework itself rather than the appliance — a loose or corroded fitting on the heating flow or return, or on a combi's cold or hot water connection. Turning the boiler off does not always stop this kind of leak, and the main stopcock only stops one on a combi's water connection.
Related symptoms
What to check, in order
1. Turn the boiler off and catch the water
Switch the boiler off at its own switch, or at the consumer unit instead if water has reached that switch or its wiring. Put a bowl or towels under the drip, and keep the water away from any sockets below it.
2. Turn the water off at the main stopcock
Close the main stopcock if you can reach it safely. That stops a leak on a combi's water connections, but a leak on the heating pipes can keep dripping, so keep catching it. Leave every valve under the boiler alone: one carries gas.
3. Find where the water is coming from without opening the casing
Look, do not dismantle. A drip from the pressure-relief pipe where it leaves through the outside wall is a pressure fault; water pooling under the casing is an internal part; water at a joint below or behind the boiler is usually pipework.
4. Book a visit and say where the water appears
Treat it as urgent if the leak is near electrics, growing, or still running with the stopcock off, stuck or unreachable; a small drip you have caught and contained can be booked promptly. Report where the water appears and any pressure gauge reading.
What not to do
- Turn the boiler off at its own switch (at the consumer unit instead, if water has reached that switch) and turn the water off at the main stopcock if you can reach it safely — don't leave it running while it leaks
- Don't ignore water anywhere near the boiler's electrics or wiring — treat that as urgent, not routine
- Don't tighten fittings or attempt a repair on the boiler's internals yourself — what's behind a gas boiler's casing is gas work, which the law allows only someone competent to do, and any engineer you pay to do it must be Gas Safe registered
- If you smell gas, or a carbon monoxide alarm sounds: don't smoke, light a flame or touch light switches, open the doors and windows, turn the gas off at the meter if you can reach it safely and it isn't in a cellar or basement, get everyone out, and call the National Gas Emergency Service free on 0800 111 999 before anything else — before calling us
When to request help
Book promptly rather than urgently for a small drip you've caught and contained; treat it as urgent if the leak is near electrics, growing, or still running with the main stopcock off, stuck or unreachable.
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Related questions
Will a leaking boiler need replacing?
General guidance, not a substitute for an on-site professional assessment — and never for the gas emergency line (0800 111 999) if you smell gas.