In an earlier contribution to this discussion,
wrote:


The basic problem is that some of the upstairs rads heat up when we
are running the CH on "water only" - from what I know of CH we have a
pump driven system, tanks in attic etc etc. Had new boiler and
cylinder installed about 18 months ago. I'm pretty sure we don't have
a non-return valve anywhere in the system, we have standard three port
valve near boiler and this is working fine.


Do you have any evidence to substantiate your statement that the 3-port
valve is working fine? This is often the culprit in cases like this -
although this would usually result in *all* radiators getting hot - not just
those upstairs.
--
Cheers,
Roger
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