BoilerFaultCodes d.21 Low
Boiler Fault Code d.21
Also written as D21 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code d.21 appears across 2 boiler brands and 25 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning 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. (As reported on Glow-worm Flexicom 24c / 28c)
Pick your boiler brand for the d.21 fix
Source
Information for the d.21 fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026