Low
d.21 on Vaillant ecoTEC sustain

This is not a fault; it simply indicates whether the comfort setting that keeps your tap water ready to heat instantly is currently turned on or off.

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

Status of the warm start for hot water: off=Function deactivated, on=Function activated and available

From this model's manual
Verified against the Vaillant Ecotec Sustain installation & service manual, page 47.
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

Press the 'i' button on the boiler display to view system statusNavigate to the warm start settings menuSelect 'on' if you want instant hot water, or 'off' if you prefer to save energy

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Related fault codes

d.20View →d.22View →

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