Fix it
Why is my boiler making a knocking noise?
Boilers knock for two very different reasons: steam collapsing where it should not exist (kettling), and pipes moving where they were never secured. One is a scale problem inside the boiler; the other is carpentry.
What it usually is
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Kettling — scale hotspots boiling water locally
Mineral scale on the heat exchanger creates spots hot enough to flash water to steam; the bubbles collapse with a rumble like a giant kettle. It reduces efficiency and stresses the exchanger.
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Trapped air in the circuit
Air pockets gurgle and tick through radiators and pipes, loudest at the top of the system. Radiators cold at the top confirm it.
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Water hammer from a fast-closing zone valve
A single sharp bang when a zone opens or closes is momentum in the pipe, not the boiler — the noise source is often rooms away from where you hear it.
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Expansion movement at pipe penetrations
Hot pipes lengthen and creak through joists and brackets as heat begins, in a fading rhythm rather than a steady knock.
What you can check yourself
Bleed the radiators from the top of the house down, then restore system pressure to the gauge band the boiler manual specifies (typically 1 to 1.5 bar cold). Note whether the noise is a run-long rumble at the boiler (kettling) or event bangs at valve changes.
When it’s a job for a pro
Kettling calls for a system flush and descale — and a check on why scale formed, which is usually water hardness the system should be dosed against. Persistent hammer needs the offending valve found and an arrestor fitted.
A power flush typically runs $400 to $900; radiator bleeding and repressurizing is a routine visit at $100 to $200 if you would rather not do it yourself.
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