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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.

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What it usually is

  1. 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.

  2. 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.

  3. 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.

  4. 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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