# Why is my boiler leaking water?

*David · Founder, Datum · Datum · Boilers*

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.

> 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.
>
> — David, Founder, Datum

## Related questions

### Is a leaking boiler dangerous?

Water and electricity don't mix — a leak reaching the boiler's electrics or your consumer unit is the real hazard, so isolate it if that's happening. A gas boiler leaking water isn't a gas risk in itself, but a leak that's been ignored can corrode gas-carrying components over time. If you ever smell gas, that's different: leave it, don't touch switches, and call 0800 111 999.

### Can I keep using my boiler if it's leaking?

Best not to. Running it circulates and loses water, drops the pressure, and can lock the boiler out anyway — while the leak quietly does damage. Turn it off and get it seen; it's a 'book it soon', not a 'live with it'.

### How much does it cost to fix a leaking boiler?

Entirely down to the cause — a valve seal is a small job, an expansion vessel mid-range, a heat exchanger usually a replace-not-repair conversation. We diagnose first and give you the fixed price, plus the repair-vs-replace numbers, before any work starts.

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Canonical page: https://www.datumclimate.co.uk/guides/boiler-leaking-water