Safety first: Turn off power and gas. Allow the appliance to cool completely before touching internal components. Risk of burns.

Check water flow: Ensure all isolation valves on the primary circuit are open and that any circulation pump (for indirect cylinder models) is running.
Bleed air: Bleed air from high points and automatic air vents in the system to restore proper circulation.
Inspect heat exchanger: Look for signs of limescale or sludge blockage. On instantaneous gas water heaters, a scaled plate heat exchanger can cause rapid overheating.
Reset thermostat: Locate the manual reset safety thermostat on the heat exchanger. Press the reset button if present. If it trips repeatedly, investigate and correct the root cause before further use.
Descale if needed: If overheating is due to scale, perform a chemical descaling of the heat exchanger using a pump and approved descaling agent.

Warning: Never bypass or short the safety thermostat. It is a critical safety device.

F8 error, appliance overheats and shuts down, casing may feel very hot

Possible Causes