BoilerFaultCodes F21 High
Boiler Fault Code F21
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code F21 appears across 2 boiler brands and 4 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler has lost water pressure and has shut down as a safety precaution to prevent damage. (As reported on Glow-worm 30ci Plus)
Pick your boiler brand for the F21 fix
Source
Information for the F21 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 27 April 2026