Glow-worm
Kettling / Localised Boiling in Heat Exchanger
Boiler making loud banging or kettling noises when heating is on
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Limescale build-up in primary heat exchanger, Sludge in system reducing flow, Pump running at too low speed, Incorrect system balancing
How to Fix / Troubleshooting
Safety: No immediate danger if the boiler is not overheating, but loud banging should be investigated. Do not ignore overheat lockouts.
- Check system pressure: Ensure pressure is correct (1.0–1.5 bar cold). Low pressure can worsen kettling.
- Bleed radiators: Remove trapped air that can cause noise and poor circulation.
- Check pump setting: If accessible, ensure the circulation pump is set to an appropriate speed for the system (usually medium or as per installer guidance).
- Engineer tasks:
- Assess the primary heat exchanger for scale and consider chemical descaling.
- Powerflush or chemically clean a sludged system.
- Rebalance radiators to ensure adequate flow through the boiler.
In hard water areas, a scale reducer or water treatment may be recommended.
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Repair Difficulty
Medium
3/5
Required Part
Primary Heat Exchanger
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