In an earlier contribution to this discussion,
AJJ wrote:

I am not sure where the return pipes join but it is certainly not in
the vicinity of the hot water tank. I suspect it is close to the
boiler but cannot check it without having the room apart. If they
join a considerable distance away from the boiler is it possible the
problem was worsened by installing a new more powerful pump?

AJJ.

What you *mustn't* have is some radiators returning one side and some the
other side of the hot water return - they must all be on the *same* side -
otherwise you can can get reverse circulation through some of the rads.

If this problem started when you changed the 3-port mid-position valve, I
would be inclined to suspect that there is some crud in the valve which is
stopping it from closing off fully - allowing *some* CH flow in the HW-only
position.
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.