Low
S.21 on Vaillant Home System

Your boiler is performing a routine start-up check by running its internal fan before beginning to heat your water.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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.

What the manual says

Manufacturer manual

Hot water handling mode: Advance fan operation

From this model's manual
Verified against the Vaillant Home System installation & service manual, page 33.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

DIY fix possible

No action requiredWait for the status code to change automatically as the heating process continues

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Related fault codes

S.20View →S.22View →S.24View →

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