S.25 Low

Glow-worm S.25 Fault Code

Also written as S25 — same fault.

The Glow-worm S.25 fault code means: your boiler is staying in a cooling-down mode because it thinks the internal temperature is still too high after the heating or hot water has turned off.. It appears on 27 Glow-worm models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: pump / fan overrun

What causes the Glow-worm S.25 fault?

This occurs when the boiler keeps its internal pump or fan running after a heating cycle to safely dissipate leftover heat. It is usually a normal safety function, but if it persists, it may indicate that the water is struggling to move through the system due to a blockage or a pump that is beginning to fail. In some cases, a faulty internal sensor might also be tricking the boiler into thinking it is hotter than it actually is.

Typical repair cost £0-120 (including labour)

DIY fix possible

Check that all radiator valves are not fully closedEnsure the boiler pressure is between 1.0 and 1.5 barWait for 5-10 minutes to see if the status clears automatically once the unit cools

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Glow-worm models that show the S.25 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Glow-worm S.25 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