Display shows H1 and heater will not fire; water may feel very hot at heater outlet.

Safety first: Allow the heater and pipework to cool before handling. Isolate power and gas if overheating is suspected.

• Check thermostat setting: Ensure the set temperature on the Certikin controller is appropriate (typically 28–30°C for pools, 37–40°C for spas).
• Verify flow: Confirm that the circulation pump is running at the correct speed and that bypass valves are correctly adjusted.
• Filter and baskets: Clean pump strainer baskets and backwash the filter to improve flow.
• Inspect outlet sensor: Check that the outlet sensor is firmly attached and not exposed to ambient air, which can cause false readings.
• Reset: Restore power and test. If H1 recurs quickly, shut down and seek professional help.

Overheating can damage the heat exchanger and is a safety risk; gas valve and control calibration must be checked by a qualified engineer.