Furnace operates but makes loud banging or knocking noises in the heat exchanger or pipework.

Safety: Noisy kettling can indicate local boiling; avoid prolonged operation until checked.

• Increase flow: Ensure all isolation valves are fully open and increase pump speed if adjustable.
• Bleed system: Bleed radiators and any automatic air vents near the furnace to remove trapped air.
• Check inhibitor: Confirm that corrosion inhibitor has been added to the system water. If not, have a professional dose the system.
• Descale heat exchanger (professional): Persistent kettling usually requires chemical descaling or replacement of the primary heat exchanger by a qualified technician.