Boiler indicates flue temperature too high, shuts down to protect heat exchanger

Safety: High flue temperatures can damage the boiler and flue. Do not continue to reset without finding the cause.

• Check system flow: Ensure all isolation valves on the boiler flow and return are open and that radiators are not all turned off. Poor water flow can cause high flue temperatures.
• Clean filters: If fitted, clean any system filter or strainer on the return to the boiler to improve circulation.
• Reset boiler: After cooling, reset the boiler and monitor. If the fault recurs, stop using the boiler.
• Professional descaling/servicing: A heating engineer should inspect and clean the primary heat exchanger, verify burner input, and test/replace the flue gas thermostat or sensor.