BoilerFaultCodesVaillant D.77 Fault Code
Also written as D77 — same fault.
The Vaillant D.77 fault code means: your boiler is temporarily reducing its power while heating your hot water cylinder because it is reaching its temperature limit too quickly.. It appears on 12 Vaillant models in our database.
Technical description: Limitation of cylinder charging in output in kW
What causes the Vaillant D.77 fault?
This occurs when the boiler is generating heat faster than your hot water cylinder can absorb it, causing the water inside the boiler to get too hot too quickly. To prevent overheating, the boiler automatically throttles its power back to a lower level to allow the heat transfer to catch up. The most common reasons are a buildup of limescale or sludge inside the heating coil, or a pump that isn't moving water fast enough through the system.
DIY fix possible
Check that the valves on the pipes leading to your hot water cylinder are fully openEnsure there is no trapped air in the cylinder coil by bleeding nearby radiatorsCheck that your hot water temperature dial on the boiler is not set excessively high
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Vaillant models that show the D.77 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.77 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
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Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026