Slant/Fin Kettling / Boiling in Heat Exchanger

Boiler makes loud banging or knocking noises when firing or shortly after

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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

Lime scale buildup inside Slant/Fin heat exchanger, Low flow through boiler, Incorrect system pressure, Air pockets in heat exchanger

How to Fix / Troubleshooting

Safety: Turn off power and allow boiler to cool. Do not ignore loud banging; it can damage the boiler.

Steps:

  • 1. Check system pressure: Ensure cold pressure is around 12–15 psi. Low pressure can promote boiling.
  • 2. Bleed air: Use air vents and bleeders to remove trapped air, especially near the boiler and high points.
  • 3. Verify circulator sizing and operation: Confirm the pump is running and appropriate for the system. Restricted flow increases local boiling.

Professional service: A technician may need to chemically descale the heat exchanger and flush the system to remove mineral deposits causing kettling.

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Repair Difficulty

Hard 4/5

Required Part

Heat Exchanger
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