BoilerFaultCodesTechnical description
Flue overheat - check combustion and flue temperature vs. return temperature (Installer numbers 7 and 3 above). The flue temperature should be no more than 10C above return temperature. Possible heat exchanger fault
What causes this fault?
This fault occurs when the internal temperature of the exhaust gases becomes dangerously high, triggering a safety shutdown to protect the appliance. The most common reason is that heat isn't being transferred effectively into your radiators, often due to a buildup of dirt or debris inside the main heat exchanger. It can also be caused by a lack of proper water flow through the boiler, which prevents the system from cooling itself down while firing.
This fault is more common in the autumn or winter when the heating is first turned back on, as stagnant sludge in the system can shift and block the narrow channels of the heat exchanger.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.