d.21 Low

Glow-worm d.21 Fault Code

Also written as D21 — same fault.

The Glow-worm d.21 fault code means: your boiler is currently indicating that the 'warm start' feature, which keeps water hot inside the unit for faster tap delivery, is being toggled or checked by the system.. It appears on 7 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: Status of the warm start for domestic hot water

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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

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

Source

Glow-worm d.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