F25 High

Glow-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 12 Glow-worm models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Condensate drain blockage

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.

Typical repair cost £120-350 (including labour)

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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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 7 July 2026