BoilerFaultCodesGlow-worm F25 Fault Code
The Glow-worm F25 fault code means: your boiler has switched itself off because the internal exhaust gases have reached a temperature that is too high, often caused by a circulation problem or a lack of water.. It appears on 9 Glow-worm models in our database.
Technical description: Flue gas overheat fault
What causes the Glow-worm F25 fault?
This fault occurs when the sensors detect that the exhaust gases leaving the boiler are getting much hotter than they should be, triggering an automatic safety shutdown. The most common reason is that heat isn't being moved away from the burner quickly enough, usually due to a lack of water pressure, a failing pump, or a blockage in the main heat exchanger.
DIY fix possible
Check the boiler pressure gauge and top it up to 1.5 bar if it is too lowEnsure all radiator valves are fully open to allow water to flowTry resetting the boiler once the system has cooled down
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Gas Safe Register →Glow-worm models that show the F25 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F25 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026