Heater makes loud banging or knocking noises during operation, sometimes called 'kettling'.

Safety first: Isolate power and allow the heater to cool before touching pipework.

• Increase flow: Ensure all valves to and from the heater are fully open and that the circulation pump is operating correctly.
• Adjust bypass: If a bypass is fitted, adjust it to send more flow through the heater, following Certikin guidelines.
• Bleed air: Open any air bleed points on the heater or high points in the system to remove trapped air.
• Descaling: If the pool is in a hard water area and the heater is older, arrange for a professional chemical clean of the heat exchanger.

Persistent kettling can damage the heat exchanger and should be addressed promptly by a technician.