D.78 Low

Vaillant D.78 Fault Code

Also written as D78 — same fault.

The Vaillant D.78 fault code means: your boiler is limiting the temperature it uses to heat your hot water tank to prevent it from getting too hot.. It appears on 5 Vaillant models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Limitation of cylinder charging temperature in c

What causes the Vaillant D.78 fault?

This issue occurs when the boiler detects that the temperature being sent to your hot water tank is too high for the cylinder's safe operating limits. It is usually caused by a faulty temperature sensor providing incorrect readings or a communication error in the wiring between the boiler and the tank. As a safety precaution, the boiler automatically scales back its heat output to prevent the water from overheating.

Typical repair cost £0-120 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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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.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026