S.11 Low

Vaillant S.11 Fault Code

Also written as S11 — same fault.

The Vaillant S.11 fault code means: your boiler is currently preparing to heat your hot water by starting up its internal air extraction system.. It appears on 40 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: DHWmode:Fanstart-up

What causes the Vaillant S.11 fault?

This is a normal status message rather than a mechanical failure, occurring every time the boiler prepares to heat your water or radiators. It indicates that the control board has sent a signal to the internal fan to begin spinning, which clears out the combustion chamber before the burner ignites. If the boiler stays on this code for a long time without firing, it may suggest the fan is struggling to reach the required speed.

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.11 fault code

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

Source

Vaillant S.11 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 7 July 2026