s plan with momo valves

[dellboy78]

Heating controls not my strong point but trying to change that! I have an s plan system with momo valves and have a question with regards to the closing of the valves. To close the heating valve the white goes on its own to CH off on the programmer. To close the hot water valve the white goes to HW off along with another wire from the cylinder stat. I just wondered why there is this extra wire from the cylinder stat for the hot water but there isn't one from the room stat for the heating? Hope that makes sense!

 

[davesparks]

Of the valves require an off signal on an S plan then you would normally expect the heating valve to have an off signal from the thermostat.
If it doesn't then the CH valve could theoretically stay open whilst the stat is not calling but the DHW is still calling.

 

[dellboy78]

I did think that but it seems to work Ok!

 

[davesparks]

What do you mean by seems to work ok?
Have you thoroughly tested to ensure that each valve opens and closes correctly in ever possible combination of programmer and thermostat demand signals?