Low
d.21 on Vaillant Home System

This is a status indicator showing whether the pre-heat function for your hot water is currently switched on or off, rather than an actual fault code.

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 Home System installation & service manual.
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

Navigate to the 'hot water' settings on your control panelSelect 'Comfort Mode' or 'Pre-heat' to toggle this setting on or off based on your preference

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Related fault codes

d.20View →d.22View →

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