Furnace makes loud banging or knocking noises when starting or stopping, sometimes called water hammer or kettling.

Safety: Turn off power and allow the furnace to cool before touching pipework.

• Bleed air: Bleed air from automatic air vents near the furnace and from radiators. Ensure the air vent caps are slightly open to allow air to escape.
• Increase flow: Check that all isolation valves are fully open and that the pump is set to an appropriate speed.
• Descale system: For hard water areas, a heating professional can perform a chemical descaling of the Viadrus heat exchanger and system using products safe for cast iron.
• Hydraulic check: If noise persists, have a technician verify pump sizing and pipe diameters to ensure adequate flow through the furnace.