Furnace makes loud banging or knocking noises when starting or stopping (kettling)

Safety: Noisy boiling can indicate overheating. Monitor pressure and temperature; shut down if pressure rises excessively.

• Check system pressure and bleed air: Ensure pressure is correct and bleed radiators and the boiler's air vents to remove trapped air.
• Verify all valves are open: Confirm that isolation valves on the boiler and nearby pipework are fully open.
• Increase pump speed: If the pump has multiple speeds, select a higher setting to improve flow.
• Descale (technician): A professional can chemically descale the primary heat exchanger using Viessmann-approved cleaners to remove limescale and sludge.