Return heating rads?? strange one

[ROSSGR]

At a job today, system chb with UNvented right next to it.
4 level house. 2 zone valves ch+dhw

When running dhw only the radiators on the 3rd floor, same level as chb, heat up. NO other levels get any heat. The ch zone valve is not passing, absolutely stone cold so it isn't passing....the pnly thing I can think of is the return leg is somehow pasing to these rads???

The return from the cylinder t's in last 1.8mt before the chb.......

any thoughts??

 

[ChrisR]

Is the bathroom off the cylinder primaries - lots are. Maybe other rads are across that too, on that floor?

As a reminder, this is reverse flow:
You want blue flow but get some red as well.
Telltale is that the "wrong" end of the rads heats up first.

 

[ROSSGR]

Cheers for the response...

From your diagram, the return from the cylnder T's in last in the configuration. SO not sure how heading to rads.

NO the bathrooms are not off the cyl primary.

|Flummoxed