How to Fix / Troubleshooting

Safety: Turn off power and allow the heater to cool completely. Do not reset repeatedly without finding the cause.

Steps:

Reset overheat stat: Locate the manual reset overheat thermostat (if fitted) on the heat exchanger. Press the reset button gently until it clicks.
Check pump operation: With power restored, call for heat and confirm the pump runs and that both flow and return pipes warm up. A very hot flow and cold return suggests no circulation.
Bleed system: Remove air from radiators and any automatic air vents. Recheck system pressure afterward.
Inspect for scale: If the heater is old or in a hard-water area, internal scaling may be severe. This requires professional descaling or heat exchanger replacement.

Warning: Do not bypass or permanently short the overheat thermostat. It is a critical safety device.

Error 11; water heater intermittently shuts down, often when hot water demand is high.

Possible Causes