CWatters wrote on Nov 23rd, 2006, 2:39pm:Interesting I assume the mechanisim is as follows...
If it looses it's charge it will fill with water when you try and restore pressure. Then, if there is any expansion (eg when the heating is switched on in cold weather) the excess is pushed out of the relief valve. Then when the heating cools down there is no pressure again.
More or less CW, water doesn't compress whereas air does.
The vessel takes up the expansion as the water gets hot.