Boiler Fault Code d.20

Also written as D20 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026

Fault code d.20 appears across 3 boiler brands and 31 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is noting that the temperature setting for your hot water storage tank has reached its maximum allowed limit, which is typically just a configuration setting rather than a breakdown. (As reported on Vaillant ecoFIT Pure 825)