Water leaking from your boiler: what to do first
Boilers
19 June 2026
Water leaking from your boiler: what to do first
Water coming out of a boiler is never something to leave until next week. Some causes are minor, some will damage the boiler, and a few carry a real risk with electrics sitting directly underneath. This page covers what to do in the first five minutes and what the leak is likely to be.
Do these four things first
Turn the heating off at the programmer.
Turn off the water supply to the boiler. On a combi that is usually the cold mains isolation valve underneath. If you cannot find it, use the main stopcock.
Keep it away from the electrics. If water is running onto or near the boiler's electrical connections or a socket, turn the power off at the consumer unit before you touch anything.
Catch it and photograph it. A container under the drip and a photo of where it is coming from genuinely speeds up the repair.
Do not attempt to take the front casing off. On a gas appliance that is a job for a Gas Safe registered engineer, and on most boilers the casing forms part of the sealed combustion chamber.
Where the water is coming from
The pressure relief valve. If water is running from a small copper pipe through an outside wall, that is the pressure relief discharge, and it is doing its job. It opens when system pressure exceeds roughly 3 bar. That usually points to a failed expansion vessel or a filling loop left slightly open, rather than a fault with the valve itself.
The expansion vessel. When the vessel loses its air charge the water has nowhere to expand into, pressure climbs as the system heats, and the relief valve dumps it outside. The classic sign is pressure that reads normal cold and climbs well past 2.5 bar hot. Re-pressurising the vessel sometimes fixes it and sometimes reveals a perished diaphragm.
Pipework joints and the pump. Corrosion or a weeping joint under the boiler is often the simplest repair on this list, and frequently the least dramatic looking.
The heat exchanger. A leak from the heat exchanger itself is the serious one. On an older boiler this is often the point where a repair stops making financial sense and replacement becomes the better decision, and any honest engineer will tell you that rather than sell you a repair that will not last.
The condensate trap or pipe. A cracked trap or a disconnected condensate pipe leaks slightly acidic water. It marks flooring and corrodes anything metal it sits on.
Not the boiler at all. Water tracking along a pipe from somewhere else and dripping at the boiler is common. So is a leaking radiator valve upstairs finding its way down. Our page on pressure that keeps dropping covers hunting those down.
Ring immediately, not tomorrow, if
Water is anywhere near electrical connections
You can smell gas as well as seeing water, in which case ring 0800 111 999 first
The leak is continuous rather than an occasional drip
The boiler is still trying to fire while leaking
What it usually costs you to wait
A weeping joint left for a month corrodes the components underneath it. Water sitting in the bottom of a boiler case rusts the chassis and can reach the circuit board, which turns a modest repair into an expensive one. Continuous discharge from the relief valve also means the system is repeatedly refilling with fresh, oxygenated water, and that accelerates corrosion in every radiator in the house.
This is one of the few faults where the cost of waiting is genuinely predictable.
Getting it stopped
We will find where the water is actually coming from before quoting, because the boiler is not always the source.
See boiler repair for how we approach it, burst and leaking pipes if it turns out to be pipework, and emergency plumber if it will not wait. We cover Manchester, Salford, Stockport and the towns around them.
Ring 0161 515 9977 and tell us where the water is coming from.
