Heater runs but water does not reach set temperature; very slow heating.

Safety first: Turn off power and gas before opening the heater.

• Verify settings: Confirm the thermostat setpoint is above current water temperature and that the correct mode (pool/spa) is selected.
• Check run time: Ensure the heater is allowed to run long enough; large pools can take many hours to heat.
• Inspect burners: Remove burner access and look for rust, debris, or spider webs. Clean burners with a soft brush and compressed air (outdoors) if needed.
• Gas supply: If other gas appliances are weak, contact your gas supplier to check pressure and meter capacity.
• Scale issues: If the heater is older and performance has declined, have a technician inspect for scale and clean or replace the heat exchanger.