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 25 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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A cover plan would handle faults like S.15: no call-out fees, no surprise bills.
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Glow-worm models that show the S.15 fault code
Click a model for the full fault code list and model-specific repair notes.
- 30cxi Repair: £120-250
- 38cxi
- Betacom2 28
- Betacom4
- Easicom 24
- Easicom3 Combi
- Easicom3 System
- Energy
- Energy 35 Store
- Energy System
- Energy2 30c
- Energy7 Combi
- Energy7 System
- Essential
- Flexicom 24c / 28c
- Flexicom cx Repair: £0-120
- Sustain 12s
- Ultimate 3 25R
- Ultimate3 Combi
- Ultimate3 System
- Ultracom 2 CXI Repair: £0-80
- Ultracom 2 SXI Repair: £0-80
- Ultracom 24cx 30cx Repair: £0-80
- Ultracom cxi Repair: £0-80
- Ultracom sxi Repair: £0-80
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.
- Glow-worm Ultracom 2 CXI manual (PDF)
- Glow-worm Betacom2 28 manual (PDF)
- Glow-worm Betacom4 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026