Boiler makes loud banging or knocking noises when starting or stopping, sometimes called kettling.

Safety: Allow the boiler to cool before touching pipework. Noisy kettling can indicate high local temperatures.

• Bleed air: Bleed radiators and the automatic air vent on the Defro boiler to remove trapped air. Ensure system pressure is correct afterward.
• Check pump speed: If the Defro pump has adjustable speeds, set it to the recommended speed for the system size to ensure adequate flow.
• Clean system filter: If a magnetic or mesh filter is installed, clean it to improve circulation.
• Descale/flush: Persistent kettling usually requires a chemical flush or descaling of the Defro primary heat exchanger and system by a professional, using manufacturer-approved chemicals.