Common causes
- A worn pump seal — one of the most common sources of a slow drip from inside the casing.
- The pressure relief valve discharging — often linked to the system pressure being too high rather than a fault with the valve itself.
- A corroded or damaged pipe — including pipework that isn't visible, hidden under floorboards or behind panels. We've traced leaks exactly like this before — see our case studies for a real example of a gas line found under floorboards in Dudley (the same principle applies to water pipework).
- A cracked heat exchanger — less common, but more serious, and something only a proper inspection can confirm.
What to do right now
- If you can see where the water's coming from and there's a nearby isolation valve, turning off the water supply to the boiler is sensible.
- Switch the boiler off if you can do so safely at the wall or the boiler's own controls.
- Don't remove any panels or attempt to inspect the internals yourself — that's not a safe DIY job.
- If you smell gas at any point, leave the property, don't operate any switches, and call the National Gas Emergency Service on 0800 111 999 before anything else.
Pricing
Leak detection and repair — first hour: £99. Anything beyond that is quoted clearly before we proceed.
What we test when we arrive
Every diagnosis follows the same discipline as our emergency callouts — our advice article on what we check on every callout covers it in full, but in short: for a leak, the first job is tracing the water to its actual source — water runs along pipes and casings, so where it drips is often not where it escapes. The engineer checks the pump and its seals, the pressure relief valve and its discharge route, the system pressure when hot and cold (a climbing hot pressure implicates the expansion vessel), and the condition of joints and pipework. Gas tightness and electrical checks are part of every visit — water and electrics in one casing deserve respect.
Likely repairs
Most leaks end with a replaced pump seal, a new pressure relief valve, a remade or replaced joint or section of pipework, or an expansion vessel recharge or replacement where over-pressure was forcing the relief valve open. A cracked heat exchanger is the rare, expensive exception — and if we find one, we'll give you an honest steer on repair versus replacement rather than a hard sell. Parts are quoted before fitting and most repairs carry a 12-month parts warranty.
Something else going on too? The Boiler Problem Finder covers every symptom we get called about, and our case studies include a gas leak traced under floorboards in Dudley, and the same tracing discipline applies to water: a Greenstar 28i Junior in Solihull weeping from its manifold, and a Worcester CDi leaking from its condensate collector.
Frequently asked questions
Should I switch a leaking boiler off?
If you can do so safely at the wall or the boiler's own controls, yes — and turn off the water supply to the boiler if there's a nearby isolation valve. Don't remove panels or investigate inside the casing yourself. If you smell gas at any point, leave the property and call 0800 111 999 first.
Is a small drip from under the boiler serious?
Worth taking seriously, yes — even a slow drip lands on electrical components and corrodes parts over time, and drips rarely fix themselves. Small leaks caught early are usually cheap repairs; the same leak left for months often isn't.
Why does my boiler only leak when the heating is on?
Heat raises the system pressure and expands the pipework, which opens up weaknesses that seal themselves when everything cools — typically a worn pump seal, a weeping joint, or the pressure relief valve lifting because pressure is climbing too high when hot.
What does a leak repair cost?
Leak detection and repair starts at £99 for the first hour. Any part — a pump seal, valve or section of pipework — is quoted before fitting, and most repairs carry a 12-month parts warranty.