Water heater displays F3 and will not ignite, often after overheating event

Safety first: Disconnect power and close gas. Allow the appliance to cool completely.

Steps:

• Check system pressure and flow: Ensure adequate water pressure and that all hot water taps and isolation valves are open. Low flow can cause overheating.
• Locate safety thermostat: On Saunier Duval water heaters, the manual reset safety thermostat is usually clipped to the primary heat exchanger. Check for a small red reset button.
• Reset thermostat: Press the reset button firmly until you feel a click. If it immediately trips again, do not keep resetting; find the cause of overheating.
• Inspect wiring: Check the connectors and wiring harness from the thermostat to the PCB for loose or damaged wires.
• Descale if needed: If the unit frequently overheats, the heat exchanger may be scaled. A professional descaling of the primary heat exchanger is recommended.

Warning: Repeated tripping indicates a serious overheating issue. Continued use without repair can damage the heat exchanger and create safety risks.