VR10 NTC on EcoTec 630 with Gledhill unvented cylinder

[kaos24]

Hi All,
we have a Vaillant EcoTec 630 with Gledhill unvented cylinder.
With the cylinders thermostat connected to the vr65 It heats hot water in the tank at the scheduled times.
I asked the installer to connect the VR10 NTC instead (disconnecting the cylinder stat in the process) & we see the tank temp but the boiler will not heat the water at the scheduled time or on cylinder boost.
Fault code f.78 appears too.
Replacing the VR10 with the cylinder stat and the boiler heats the tank once more as scheduled.
The installer insists its simply a matter of connecting the NTC or the cylinder stat and the boiler figures out the rest, but he can't explain why it refuses to heat the water when ntc is connected. He claims the f.78 is a red herring as it heats water when using the cylinder stat.
I'm dreading having engage with the grumpy Vaillant engineers and wait around all day for them to arrive at 5pm.

can anyone offer any suggestions on how to get the 630 to heat the cylinder when using the vr10 NTC?

thanks

Chris

edit to correct spelling of ecotec

 

[Dan Robinson]

Edit.

 

[kaos24]

So...
clearing the fault and redoing the installation assistant, ensuring the DHW temp is set to something above the 40 degrees listed (set to 65), the boiler started heating water.
Set the cylinder boost via vrc470f to 25 and set HW to 7am to 8pm and we should now always have hot water

only taken a year!!