BoilerFaultCodes 21:1 to 21:100 Low
Boiler Fault Code 21:1 to 21:100
Also written as 211TO21100 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 21:1 to 21:100 appears across 1 boiler brand and 1 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler is reminding you that it is due for its routine annual safety service based on the number of hours it has been running. (As reported on Viessmann Vitodens 333)
Pick your boiler brand for the 21:1 to 21:100 fix
Source
Information for the 21:1 to 21:100 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