Boilers
Why is my boiler leaking water?
3 min read · Reviewed for accuracy before publishing
Water under or inside a boiler is never normal, but the cost swings enormously with the cause. The common, cheaper ones: a perished pump or valve seal, or a weeping pressure-relief valve because system pressure is running too high (often a failing expansion vessel). More serious is corroded internal pipework — and the one that usually ends a boiler's life is a cracked heat exchanger. If water is actively dripping, especially near the electrics, switch the boiler off at the fused spur, catch the water, and book an engineer before a slow leak finds your floors and ceilings.
The fixable causes
Seals on the pump or a valve perish with age — a contained, mid-range repair. A pressure-relief valve dripping outside through its little copper pipe usually means system pressure is too high or the expansion vessel has lost its charge; re-pressurising and recharging the vessel sorts most of these. None of them fix themselves, but none of them are the end of the boiler.
The serious one
A cracked or corroded heat exchanger leaks from the heart of the boiler, and on an older unit the repair cost rarely justifies itself against a new boiler with a fresh warranty. An engineer confirms it quickly with a proper inspection — and we always quote you both the repair and the replacement so the choice is yours, with the real numbers in front of you.
What to do this minute
If it's dripping onto or near electrics, turn the boiler off and isolate it. Mop up standing water. Don't keep topping up and running a boiler that's visibly leaking — you're just feeding the leak. Send us the model and a photo of where the water's coming from and we'll tell you what you're likely looking at before anyone's booked.
David founded Datum after years on the tools across heating and plumbing. He writes the boiler and cost guides from what actually happens on real installs.