Before I start, I have a fairly intimate knowlege of my system, having
done a large amount of work on it myself. I think I understand what
the problem is, but I don't know how to resolve it. So here goes:

Boiler in cellar. Output spilts just after the boiler by means of
Horstmann flowshare valve. Heating side splits and goes in separate
directions. Water side heads off upstairs alongside one part of the
heating circuit. The returns for these last 2 come back downstairs
side by side join the return from the other part of the heating
circuit before finally before going back into the boiler.

During the summer I have noticed the radiators getting hot while the
water programmer is demanding heat from the boiler. This at times
drives me mad. I am having a purge on wasting energy at the mo and
sorting this is high on the list.

I had to replace the programmer and flowshare a while back and thought
this may also solve the problem if the old valve perhaps wasn't
closing 100%, but still got exactly the same symptoms with the new
valve. So I'm sure its not the valve and the program controller
connections.

What I have since discovered is that the return pipe from the upstairs
heating is at roughly the same temperature (to touch) as the return
from the water, while the heating flow to upstairs is notably cooler.
This suggests that some of the water retunring from the cylinder tank
flows back up the heating return, rather than all of it going into the
boiler. What I can't fathom is where this is flowing to as the other
end of the circuit should be blocked by the flowshare valve, so this
shouldn't happen. It just doesn't make sense.

Has anyone else come across this. Can you fit some sort of 22mm one
way valve to stop return going the wrong way up an ajoining return
pipe on a different circuit, rather than back into the boiler?

Cheers