"Roger" wrote in message
k...
The message om
from "Dave Liquorice" contains these words:

Ah but is that air actually being sucked in down the vent or is it the
air relased from the water when it is heated?

I am convinced

Roger, be convinced all you like. There are packaged tank cylinder setups
with this sort of head. If what you sad was right they would all suck in
air.