Boilers
Why is my boiler leaking water?
3 min read · Last updated · 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: worn seals, or a pressure-relief valve letting water out because system pressure is too high — Vaillant calls excessive pressure one of the most common causes, and lists a faulty expansion vessel or passing filling loop. More serious is corrosion, and Vaillant says a heat exchanger can fail or crack over time, especially in older boilers. 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
Vaillant says seals can loosen or break over time and leak — a contained, mid-range repair. A pressure-relief valve letting water out usually means system pressure is too high, and Vaillant says it begins to release water above 3 bar; a faulty expansion vessel or passing filling loop can be behind it. 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 may not justify 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.
“Water where it should not be is never a keep-an-eye-on-it. Isolate it, catch it, call — the leak you ignore is the ceiling you replace.”
Sources
- Vaillant — Is my boiler leaking water? (excessive pressure is among the most common causes; the pressure relief valve lets water out above 3 bar; a faulty expansion vessel or passing filling loop, corrosion, worn seals and a cracked heat exchanger are other causes; turn off the water supply and heating and call a Gas Safe registered engineer; accessed 19 Oct 2026)
- National Gas — Emergency contacts (gas emergency line 0800 111 999; accessed 19 Oct 2026)
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.