BoilerFaultCodes S.021 Low
Boiler Fault Code S.021
Also written as S021 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code S.021 appears across 1 boiler brand 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 is currently heating the water stored in your hot water cylinder, which is a normal part of its operational cycle. (As reported on Vaillant Turbomax 242/282 1E)
Pick your boiler brand for the S.021 fix
Source
Information for the S.021 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 31 August 2026