BoilerFaultCodesVaillant D.25 Fault Code
Also written as D25 — same fault.
The Vaillant D.25 fault code means: your boiler is currently busy heating your hot water because your smart thermostat or external controller has told it to do so.. It appears on 37 Vaillant models in our database.
Technical description: Hot water activation via eBus control
What causes the Vaillant D.25 fault?
This code appears when your external smart controls or cylinder thermostat send a digital signal asking the boiler to focus on heating your hot water. It is a status message rather than a mechanical failure, confirming that the internal valve has successfully switched over to the water heating position. Your heating may pause briefly during this time because the boiler is prioritising your taps or hot water tank.
DIY fix possible
Check if your hot water timer is active on your external thermostatWait for the hot water tank to reach its set temperatureNo action is required as this is a status message, not a system failure
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Vaillant models that show the D.25 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoMAX
- ecoMAX 613
- ecoMAX 618
- ecoMAX 622
- ecoMAX 635
- ecoMAX 824E
- ecoMAX 828E
- Ecomax Condensing 600/800 series
- ecoMAX System 612
- ecoTEC
- ecoTEC 65kW
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 840
- ecoTEC plus aquaPLUS
- ecoTEC Pro / ecoTEC Plus
- ecoTEC sustain
- Home Combi
- Home System
- Thermocompact
- Turbomax 824/828
- Turbomax Plus Repair: £120-220
- Turbomax Plus 824/828E
- Turbomax Plus 828
- Turbomax Plus 828 E
- Turbomax Plus VUW 242E
- Turbomax Plus VUW 242EH
Source
Vaillant D.25 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 ecoMAX manual (PDF)
- Vaillant ecoMAX System 612 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026