Heater makes loud banging or knocking noises during operation.

Safety first: Turn off power and allow the heater to cool before touching any components. Hot water and steam can cause burns.

Step-by-step checks:

• Check flow and valves: Ensure the pump is running at the correct speed and that valves to and from the heater are fully open. Adjust any bypass valve to increase flow through the heater.
• Bleed air: If the system has air bleeders near the heater, open them to release trapped air. Air pockets can cause localized boiling.
• Inspect filter: Clean or backwash the pool filter to restore proper flow.
• Evaluate for scale: Persistent kettling noises often indicate internal scale. A professional can perform a chemical descaling of the Pahlen heat exchanger or recommend replacement if severely fouled.

When to stop: Do not continue operating a heater that is banging or boiling; this can damage the heat exchanger. Contact a technician if flow adjustments do not resolve the noise.