S.21 Low

Glow-worm S.21 Fault Code

Also written as S21 — same fault.

The Glow-worm S.21 fault code means: your boiler is performing a routine check of the internal fan system before it begins the heating process.. It appears on 25 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: fan pre-run

What causes the Glow-worm S.21 fault?

This code appears when the boiler's control board sends a signal to the fan to spin up and clear any remaining gases from the combustion chamber. It is a standard safety procedure to ensure the system is clear and safe before the burner ignites. If the code stays on the screen for a long time, it suggests the fan is struggling to reach the required speed to allow the ignition sequence to continue.

DIY fix possible

No action is required as this is a normal status message.Wait a few moments for the boiler to complete its startup sequence.If the code stays on screen for a long time without the boiler firing, try a simple reset.

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Glow-worm models that show the S.21 fault code

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

Source

Glow-worm S.21 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 7 July 2026