Boiler Makes a Banging Noise When It Starts
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Post a job — we'll find you an engineer →A delayed ignition bang (loud thud when the burner lights) can crack the heat exchanger over time and must be investigated by a Gas Safe engineer promptly.
A banging or thudding noise when a boiler starts up is almost always caused by delayed ignition — where gas builds up before igniting, then lights with a small explosion. It can also be caused by a dirty burner or a faulty ignition electrode. This is not normal and needs prompt attention from a Gas Safe engineer.
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Most likely cause & what to check
Note exactly when the bang happens — is it the moment the burner lights, or a few seconds after the boiler has been running? A bang at the instant of ignition suggests ignition issues; a bang after a few minutes suggests water hammer or air in the system.
If the bang happens at ignition, stop using the boiler and call a Gas Safe engineer. Delayed ignition can crack the heat exchanger — a very expensive repair (£500+) — and repeated instances make it progressively worse.
If the bang happens after the boiler has been running for a few minutes and you also hear gurgling, this is likely air in the system combined with water hammer. Bleed all radiators and repressurise the system.
Check the pressure gauge — low pressure can cause the boiler to fire erratically and make unusual sounds. Repressurise to 1.5 bar if below 1 bar.
A Gas Safe engineer will clean the burner, inspect the ignition electrode for wear, and check the gas valve operation. A burner clean costs £80–160 as part of a service.
If the bang is followed by the boiler locking out and showing a fault code, note the code and look it up — many fault codes after ignition attempts point to the gas valve or electrode needing replacement (£150–350).
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