BoilerFaultCodesGlow-worm F25 Fault Code
The Glow-worm F25 fault code means: your boiler has switched itself off because the waste gases are getting too hot, which is usually caused by a lack of water circulating through the system.. It appears on 4 Glow-worm models in our database.
Technical description: Flue gas temperature too high
What causes the Glow-worm F25 fault?
This fault occurs when the internal sensors detect that the exhaust gases are getting dangerously hot, triggering a safety shutdown to prevent damage. The most common reason is that heat isn't being carried away from the burner quickly enough, usually because the water isn't circulating properly or there is a blockage within the boiler's pipework.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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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 31 August 2026