Heater makes loud banging or popping noises during operation.

How to Fix / Troubleshooting

Safety first: Turn off power and allow the heater to cool before touching plumbing.

Increase water flow: Ensure pump is on high speed (if variable speed) and that filter is clean. Open any bypass valve slightly less to send more water through the heater.
Bleed air: Open the air relief valve on the filter until water flows steadily to remove trapped air from the system.
Check for scale: If the pool has hard water and the heater is older, scale may be present. A professional can chemically descale the heat exchanger.

Persistent banging indicates risk of damage; schedule service to inspect and descale the heater.