BoilerFaultCodesVaillant D.20 Fault Code
Also written as D20 — same fault.
The Vaillant D.20 fault code means: your boiler has reached the maximum allowed temperature for heating your hot water cylinder and is pausing operation as a standard safety limit.. It appears on 19 Vaillant models in our database.
Technical description: Maximum set value for target cylinder
What causes the Vaillant D.20 fault?
This occurs when your boiler reaches its temperature limit while heating your hot water cylinder, causing it to pause temporarily to prevent overheating. It often happens because the boiler is producing heat faster than the cylinder can absorb it, which is typical once the water in the tank is nearly at its target temperature. The boiler is simply regulating itself rather than failing.
DIY fix possible
Check the hot water temperature setting on the boiler control panelReduce the target cylinder temperature if it is set unnecessarily highWait for the system to cool down naturally before the boiler resumes
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Vaillant models that show the D.20 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £0-90
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825 Repair: £80-150
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoTEC
- ecoTEC 65kW
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 840
- ecoTEC Pro / ecoTEC Plus
- ecoTEC sustain
- Home Combi Repair: £120-250
- Home System
Source
Vaillant D.20 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 manual (PDF)
- Vaillant ecoTEC 65kW manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026