Dual System

[Dreamon100]

My issue is as follows.
We have a combi boiler and a dual CH system one part underfloor &one part radiators. These are controlled by two thermostats .
I’m sure the systems used to operate separately ie the underfloor or the radiators could be on or both depending on the thermostats.
Now we’ve noticed that the radiators are coming on as well when the underfloor thermostat operates. If the radiator thermostat operates the underfloor (pump) doesn’t come on.
I can only find one zone valve which feeds the radiators although l have been told this is powered by the underfloor thermostat.

My question is how SHOULD a system like this be controlled?

 

[SJB060685]

Assuming it's a high temperature system then you should have a zone valve for both CH and UFH, each controlled by a stat.
It's most likely the CH valve has failed open or part open.

 

[Dreamon100]

My problem is no one can find a second zone valve.
Is it feasible that the UFH stat simply switches the UFH pump on and off?

 

[SJB060685]

It could be wired in such a way yes.