d.21 Low

Vaillant d.21 Fault Code

Also written as D21 — same fault.

The Vaillant d.21 fault code means: this code is simply an information message telling you whether your boiler's 'fast hot water' feature is currently turned on or off, rather than signaling a fault.. It appears on 16 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: Status of the warm start for hot water: off=Function deactivated, on=Function activated and available

DIY fix possible

Locate the 'warm start' or 'comfort' button on your boiler's control panelPress the button to toggle the setting between 'on' and 'off' based on your preferenceNote that no further action is required as this is a status screen and not an error

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Vaillant models that show the d.21 fault code

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

Source

Vaillant d.21 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 27 April 2026