S plan with HW OFF?

[Neil45]

Hi
I would like to replace my ST9400 and remote thermostat with a T6R-HW.
As far as I can tell I have an s plan system (I.e. it seems to have two valves) but in the st9400 controller I see that the pin 1 hw off is connected but all the diagrams I can find only have hw off on a y plan. Can anyone explain pls?

 

[flameport]

Several explanations, the most likely being that it was a Y plan originally and was converted to S plan at some point such as when an unvented cylinder was installed.

In any case the presence of additional wires doesn't change anything - if the system works properly now, connecting the new controller in the same way will result in it still working correctly.

You will still need to link the switch wires for the existing thermostat so that the heating will work.

 

[Neil45]

Thanks flameport that’s what I was guessing at, but nice to have it confirmed.