S.20 Low

Boiler Fault Code S.20

Also written as S20 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code S.20 appears across 3 boiler brands and 83 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is simply letting you know that the 'warm start' feature is active, which keeps a small amount of water pre-heated so you get hot water at your taps more quickly. (As reported on Vaillant ecoTEC Pro / ecoTEC Plus)

Pick your boiler brand for the S.20 fix

Glow-worm S.20 27 models Heatline S.20 7 models Vaillant S.20 49 models

Source

Information for the S.20 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