BoilerFaultCodesGlow-worm S.15 Fault Code
Also written as S15 — same fault.
The Glow-worm S.15 fault code means: your boiler is staying in a cooling-down phase for longer than expected because it is trying to move excess heat away from the internal components.. It appears on 9 Glow-worm models in our database.
Technical description: pump / fan overrun
What causes the Glow-worm S.15 fault?
This occurs when the boiler has reached a high internal temperature and the pump or fan must continue running to dissipate that heat safely. The most common reasons are a blockage in the system restricting water flow, a pump that is beginning to fail, or a buildup of limescale inside the heat exchanger. Essentially, the boiler is struggling to push the hot water away from itself quickly enough after a cycle.
This issue is more frequent in summer when the central heating is rarely used, as pumps are more likely to seize or become sluggish after long periods of inactivity.
DIY fix possible
Check that at least one radiator is turned on fully (TRV set to max) to allow heat dissipationEnsure the system pressure is between 1.0 and 1.5 barWait up to 10 minutes for the boiler to complete its natural cooling cycleBleed radiators to ensure there is no trapped air affecting circulation
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Gas Safe Register →Glow-worm models that show the S.15 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm S.15 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