It has a float in it which drops when air fills the bottle, opening up the
valve, and letting air out.

--
Tony Collins



"Nigel Percy" wrote in message
...
Hi,

Right all you knowledgeable persons. I moved house and am trying to get to
know
the various bits & pieces. What is the arrowed component in the following
picture

http://www.btinternet.com/~nigel.percy/heating.jpg

What's it for? How does it work?

Cheers

Nige