Boiler makes loud banging or knocking noises (kettling) during operation, especially when heating up.

Safety first: No immediate danger if there is no leak, but persistent kettling can damage the boiler. Allow the boiler to cool before touching any pipes.

Step-by-step checks:

• 1. Check system pressure and bleed air: Ensure pressure is around 1.0–1.5 bar cold and bleed radiators and the boiler air vent to remove trapped air.
• 2. Verify pump speed: On multi-speed pumps, set the pump to a higher speed to improve circulation, following Viadrus recommendations.
• 3. Flush system (basic): If radiators are cold at the bottom and hot at the top, sludge is likely. A full chemical flush and inhibitor dosing should be carried out by a professional.
• 4. Descale heat exchanger: In hard water areas, the primary heat exchanger may need descaling. This usually requires isolating and circulating a descaling solution through the boiler, which should be done by a heating engineer.

When to stop: If banging is severe or accompanied by overheat trips, stop using the boiler and arrange professional service.