BoilerFaultCodesGlow-worm S.27 Fault Code
Also written as S27 — same fault.
The Glow-worm S.27 fault code means: your boiler is continuing to run the internal pump to cool down after use, which is a normal process, but it may be taking longer than usual or indicating an internal circulation issue.. It appears on 9 Glow-worm models in our database.
Technical description: pump overrun
What causes the Glow-worm S.27 fault?
This occurs when the boiler detects that the internal temperature is remaining too high after the burners have switched off, prompting the pump to stay running to move heat away. The most common reason is a restriction in water flow or a pump that is beginning to struggle, meaning the system cannot shed heat as quickly as it should. Occasionally, it can be caused by a sensor incorrectly reporting that the heat exchanger is still hot.
This is more common in summer when radiators are turned off, as the boiler has fewer paths to dissipate heat after providing hot water.
DIY fix possible
Check that all radiator valves are open to ensure good circulationMonitor the display to see if the code clears once the cooling cycle is completeEnsure there is no air trapped in the system by bleeding radiators
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Gas Safe Register →Glow-worm models that show the S.27 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm S.27 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