BoilerFaultCodesGlow-worm S.20 Fault Code
Also written as S20 — same fault.
The Glow-worm S.20 fault code means: your boiler is temporarily heating the internal tank to ensure hot water is ready for use, which is a normal operating status rather than a fault.. It appears on 9 Glow-worm models in our database.
Technical description: domestic hot water cycling mode active
What causes the Glow-worm S.20 fault?
This status occurs when the boiler detects that the temperature in the integrated hot water storage tank has dropped below the required level. The boiler temporarily stops circulating heat to your radiators to focus all its energy on reheating the internal cylinder. Once the water inside the tank reaches the correct temperature, the boiler will automatically return to its normal heating schedule.
DIY fix possible
Check your hot water cylinder thermostat settingsEnsure your heating and hot water timers are set correctlyMonitor the boiler to see if it moves to a standard heating or standby mode once the water is warm
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Gas Safe Register →Glow-worm models that show the S.20 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm S.20 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