Hi Darren,

On Sunday, 25 May 2014 11:47:41 UTC+1, wrote:

*Obviously the tank was full of air when I first connect it up and I am
wondering if *I need to bleed the air from the water side of the membrane to
fill it up with water? There seems to be no way to do it, and no suggestion
it should be done, so I will presumably just contract legionaire's shortly?

Your premise regarding the initial state is incorrect. If pre-charged, which is the state that all expansion vessels are usually supplied in, there is very little air on the water side of the membrane hence they don't need to be bled.

Take a look at the following cut-away:

http://diy.blogoverflow.com/files/20...nsion-Tank.jpg

Of course, there can be no pressures involved in such a situation however it might help visualise how the diaphragm sits when pre-charged i.e. it doesn't sit midway down the vessel as you might otherwise think (if it did it'd have to stretch to accommodate the necessary movement which would do nothing for longevity).

The following diagram shows the subsequent accommodation of expansion:

http://docs.engineeringtoolbox.com/d...nsion-tank.png

Mathew