I've just moved my hot water cylinder to the loft. The cold header tank
is located above the top of the hot water cylinder tank, and the
connection from the lower part of the cold header tank to the cylinder
uses 22mm all the way, although has 3 90degree elbows.

I have a problem with air being sucked in the vent pipe to the ho****er
when the bath tap is fully open. I confirmed this by hold the end of
the vent pipe in a glass of water; all the water was eventually sucked
up when the hot bath tap was fully open

After reviewing several posts to this news group I read that the most
obvious problem is the poor flow from the cold header to the cylinder.
Most posts seem to suggest upgrading this pipework to 28mm, reducing
the number of elbows with bends, etc...

I have a couple of questions I hope someone can help with:

1: I didn't have this problem before when the cold header was in the
loft, and the hot cylinder was on the landing. am I right to think that
the flow from the cold header to the cylinder was under much higher
pressure because of the head of water. now both cold header and hot
cylinder are in the loft the head of water has been reduced
significantly from what is was before.

2: even if I upgrade the pipe to 28mm, I'm not aware of a fitting that
connects 28mm pipe to the 1inch cylinder connector. would I have to
convert the 28mm down to 22mm to connect it to the cylinder, and
therefore wouldn't this still be the bottleneck?

thanks for any advice or links to sites that sell the appropriate
connectors