S.109 Low

Glow-worm S.109 Fault Code

Also written as S109 — same fault.

The Glow-worm S.109 fault code means: your boiler has entered a temporary sleep mode and is currently waiting for a signal from your thermostat or hot water timer to begin heating.. It appears on 14 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: Product's standby mode activated

DIY fix possible

Check your room thermostat and ensure it is set to a temperature higher than the current room temperature.Verify that your heating timer or programmer is set to an 'On' period.Check if your hot water cylinder thermostat is calling for heat.

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Glow-worm models that show the S.109 fault code

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

Source

Glow-worm S.109 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