"Christian McArdle" wrote in message
t...

Be very careful. A gravity fed (not just circulated) primary system
requires
an unrestricted feed line too, as well as an unrestricted vent.
Christian.



Good point. The valve is fitted in the airing cupboard in the return line as
it leaves the HW cylinder in the airing cupboard before it tees into the
feed/return line which then runs on to the boiler. Thus the gravity return
is closed off by the valve but the feed and vent is still open.