Flue Overheat
The flue temperature has exceeded safe limits. The flue temperature should be no more than 10°C above the return temperature — a higher reading suggests heat exchanger issues.
Step-by-Step Fix Guide
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Step 1
Turn off the boiler
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Step 2 Engineer only
Call a Gas Safe engineer to carry out a combustion analysis and check the heat exchanger for scale or blockage
Work on gas appliances must be carried out by a Gas Safe registered engineer. The steps above cover safe checks a homeowner can perform - any internal component replacement requires a qualified professional. If in doubt, always call an engineer.