Gas Safe: Gas work, flue work, and PCB replacement on a gas boiler must be carried out by a Gas Safe registered engineer. Homeowners can check obvious items such as heating pressure, condensate overflow, and whether a control is unplugged — then call an engineer.
Always confirm the part number against the boiler GC number and the installation manual before ordering.
Overview
F.78 means a UK link box (or similar external control interface) is connected, but the domestic hot-water NTC is not bridged or not fitted. The PCB is waiting for a DHW outlet sensor that is not there.
Vaillant meaning: Interruption to DHW outlet sensor at external controller
Possible causes
- UK link box connected without the DHW NTC bridge
- DHW NTC unplugged on a system that needs it
- Wrong interface wiring after a control change
Possible solutions
- If no external DHW sensor is required, fit the manufacturer’s bridge / link on the DHW NTC terminals as shown in the link-box instructions.
- If a DHW outlet NTC is required, fit and plug it in, then check the lead.
- Remove or rewire the link box if it should not be on this boiler.
Possible parts needed
- DHW NTC bridge / link plug for the UK link box
- DHW outlet NTC if the design uses one
- UK link box only if that accessory is actually required