Heater shuts down with F5 error; may occur after running for some time, especially in warm weather or in a plant room with poor ventilation.

Safety first: Turn off power and allow the heater to cool fully before touching internal components. Surfaces may be very hot.

• Check water flow: Confirm that the circulation pump is running and that valves to and from the heater are fully open. Ensure the filter is clean and backwashed.
• Bypass check: If a bypass valve is installed around the Certikin heater, verify that it is correctly adjusted to maintain adequate flow through the heater.
• Ventilation: Ensure the plant room has sufficient ventilation openings and that they are not blocked. Remove stored items that restrict airflow around the heater.
• Inspect high limit thermostat: Locate the manual reset high limit (if fitted) on the heat exchanger. Press the reset button if present. Check for loose wiring connections.
• Restart: After cooling and checks, restore power and test. If F5 recurs quickly, stop using the heater.

Persistent F5 errors may indicate internal scaling or a failing high limit thermostat and should be investigated by a pool heating specialist.