I know shower pump questions come along all of the time, but I can't seem to
get my head round this scenario.

I have a standard (is there such a thing?) hot water cylinder in my airing
cupboard. This has its cold feed from the storage tank in the loft, probably
some 2m+ above the inlet to the cylinder. The cylinder is a new one which
has a flat top, not domed, and has a jacket (not that foam dipped look).

If you have a pump, mounted on the floor of the airing cupboard, and this is
sucking from an Essex flange, if the cold water supply rate to the cylinder
is less than the hot water pump rate, won't this result in problems? Surely
the level of water in the cylinder will lower and air will be drawn in
through the vent, and that the domestic hw will also be cut off when the
vent empties?!

I can't see how this always isn't a problem, or does the height of the
coldstorage 'always' provide a higher flow rate than a 1.4-1.8 bar pump?

I'm sorry for being so thick!