BoilerFaultCodesVaillant d.21 Fault Code
Also written as D21 — same fault.
The Vaillant d.21 fault code means: your boiler is simply indicating that the 'warm start' feature, which keeps water hot for faster delivery to your taps, is currently active or has been recently adjusted.. It appears on 18 Vaillant models in our database.
Technical description: Status of the warm start for hot water
DIY fix possible
Navigate to the boiler display menuLocate the 'Warmstart' or 'Comfort' settingToggle the setting to 'On' or 'Off' depending on your preference to see if the status code clears from view
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Vaillant models that show the d.21 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoTEC plus aquaPLUS Repair: £120-280
- ecoTEC sustain
- Home Combi
- Home System
- Thermocompact Repair: £120-250
- Turbomax 824/828 Repair: £120-250
- Turbomax Plus Repair: £120-250
- Turbomax Plus 824/828E Repair: £120-250
- Turbomax Plus 828 Repair: £120-250
- Turbomax Plus 828 E Repair: £120-250
- Turbomax Plus VUW 242E Repair: £120-250
- Turbomax Plus VUW 242EH Repair: £120-250
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.
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoTEC plus aquaPLUS manual (PDF)
- Vaillant ecoTEC sustain manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026