"John Rumm" wrote in message
...
wrote:

just to clarify, all I've done is move the hot water cylinder from the
landing to the loft. in the loft I have built a wooden platform and sat
the cold header tank (that was always in the loft) on to the platform.
the hot water cylinder sits on the joists (on a think piece of wood to
spread the load over 6 joists) and the bottom of the cold header is now
just above the top of the hot water cylinder.

the head of water above the top of the hot water cylinder must be
around18 inches (essentially this is the height of the cold water tank
since the water in the cold header is only a few inches from the top
when full). the head of water for the hot bath tap must now be
something like 8 feet (to the top of the water in the cold header)

I'm still unsure if this vast difference in the head of water is the
cause of the problem or not. all the plumbing is 22mm, right the way to
the bath tap.

When we moved in here, the hot water system was very similar to what you
have described here, and yes, it did suck gob loads of air into the
system!

One simple fix you could try would be to arrange the hot out from the
cylinder in the way some shower pump manufacturers suggest for setups that
are not using dedicated flanges etc:

Yep. If the uprating of the cold feed does not do it a £15 Surrey flange
will. If a 1" tapping on the cylidner draw-off then get a 1" Surrey Flange.