F.78 — Interruption to DHW outlet sensor at external controller

What does F.78 mean?

This fault indicates an interruption with the Domestic Hot Water (DHW) outlet sensor, particularly when an external controller is connected using a UK link box, and the hot water NTC is not correctly bridged.

• UK link box is connected, but the hot water NTC sensor is not bridged.
• Wiring error or interruption at the DHW outlet sensor connected to the external controller.

Click steps to track your progress.

1. 1

   Press the fault clearance key on the boiler to reset it.

2. 2

   If you have an external controller and UK link box, verify the wiring according to the installation instructions.

3. 3

   If the fault persists, contact Vaillant Service Solutions to inspect the wiring and bridging of the hot water NTC.