"Christian McArdle" wrote in message
et...
What puzzles me on this is if, say the HW circuit was being pumped by
its
own pump and CH was off, why there would be no circulation on the CH
circuit
without the a zone valve?

There won't be a substantial pressure difference over the two ends of the
unpumped circuit, so flow would be non-existent or very slight, easily
stiffled by a flap type non-return valve (or the 2 port zone valve, of
course!).

After all this help I am temped to put in a second pump and go to a fully
pumped system (second option for me would to go to C-Plan)

Benefit of second pump is that it leaves the CH circuit - as is, and should
be cheaper than S-Plan as it only needs a pump (and I might be able to get
my hands on one!!), which should be cheaper and easier than having to move
the pump AND need to fit 2x 2ports AND a bypass.

The only thing that is puzzeling me is the "anti gravity valve" - what does
one look like, and where can you buy one from. I have looked at screwfix,
discountheating, BES etc and cannot fine what I need. - can anyone point me
in the right direction. I would need a 28mm fitting.

thanks