S.021 Low

Vaillant S.021 Fault Code

Also written as S021 — same fault.

The Vaillant S.021 fault code means: your boiler is currently heating the water stored in your hot water cylinder, which is a normal part of its operational cycle.. It appears on 4 Vaillant models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: The domestic hot water cylinder charging is active and the fan starts.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Vaillant models that show the S.021 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Vaillant S.021 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

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