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 28 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
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.25 fault code
Click a model for the full fault code list and model-specific repair notes.
- aquaPLUS
- Atmomax
- Combi Compact 242EB
- Combi Compact 282EB
- ecoFIT pure
- ecoFIT Pure 412
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 835
- Ecomax Condensing 600/800 series
- ecoTEC
- ecoTEC plus
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 65
- ecoTEC Plus 832
- ecoTEC Plus 836
- ecoTEC Plus 840
- ecoTEC Plus 938
- ecoTEC plus Aquaplus
- ecoTEC plus Regular
- ecoTEC plus System
- ecoTEC sustain
- Home System
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.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026