Furnace makes loud banging or knocking noises when starting or stopping

Safety first: Be cautious around hot pipework.

• Bleed system: Remove air from radiators and high points to reduce sudden flow changes.
• Adjust pump speed: Reduce pump speed if set too high for the system.
• Secure pipework: Clip and support loose pipes to prevent movement when valves open or close.
• Check valves: A plumber can fit anti-water-hammer devices or adjust fast-closing valves that cause banging.