D.75 Medium

Vaillant D.75 Fault Code

Also written as D75 — same fault.

The Vaillant D.75 fault code means: your boiler is taking longer than expected to heat up your hot water tank, causing the system to stop as a precaution.. It appears on 20 Vaillant models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Maximum charging time for a storage without own controller

What causes the Vaillant D.75 fault?

This fault happens when the boiler fails to heat your hot water cylinder within a set time limit, usually because the internal sensors haven't detected a sufficient rise in temperature. The most common cause is a faulty sensor giving an incorrect reading, or a mechanical failure in the diverter valve which prevents heat from actually reaching the water tank.

Typical repair cost £100-250 (including labour)

DIY fix possible

Check that the valves on your hot water cylinder are fully openEnsure your thermostat is calling for hot waterTry resetting the boiler to see if the error clears

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Vaillant models that show the D.75 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Vaillant D.75 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 7 July 2026