BoilerFaultCodesVaillant D.78 Fault Code
Also written as D78 — same fault.
The Vaillant D.78 fault code means: your boiler is limiting the temperature of the water being sent to your hot water cylinder, which typically means the water won't get as hot as usual.. It appears on 4 Vaillant models in our database.
Technical description: Limitation of cylinder charging temperature in celsius
What causes the Vaillant D.78 fault?
This fault occurs when the boiler detects that it cannot safely heat your hot water cylinder to the requested temperature, often due to a conflict between the boiler's settings and the cylinder's actual heat demand. It is typically caused by a faulty temperature sensor or a thermostat that is sending inaccurate data to the boiler's control board.
DIY fix possible
Locate the hot water temperature dial or digital setting on the boiler control panelAdjust the hot water target temperature slightly to see if the status clearsCheck if the hot water cylinder thermostat is set higher than the boiler's output limit
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Vaillant models that show the D.78 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.78 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
- Vaillant ecoTEC manual (PDF)
- Vaillant ecoTEC 65kW manual (PDF)
- Vaillant ecoTEC Pro / ecoTEC Plus manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026