Furnace makes loud banging or knocking noises when starting or stopping

Safety: Allow system to cool before touching pipework.

Steps:

• Bleed air: Bleed all radiators and high points to remove trapped air that can cause banging.
• Check valves: Ensure motorised valves and thermostatic radiator valves operate smoothly and are not slamming shut.
• Reduce temperature: Lower the maximum flow temperature slightly to reduce expansion stress.
• Secure pipes: Add or tighten pipe clips where pipes move and hit structures.

Professional help: Persistent banging may require installation of shock absorbers or system redesign.