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S.21 on Vaillant ecoTEC Plus 840

Your boiler is briefly checking its fan and clearing the system before it starts heating your hot 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.

Technical description

Hotwaterhandlingmode:Advancefanoperation

What causes this fault?

This is a normal part of the boiler's startup process where the internal fan spins up to clear the combustion chamber before the burner ignites for hot water. It isn't a fault, but rather a brief preparation phase to ensure the system is safe and ready to heat your water efficiently. You will usually see this appear for just a few seconds before the boiler begins its normal operation.

DIY fix possible

This fault may be fixable without an engineer. Check your manual first.

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Related fault codes

S.20View →S.24View →

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