I assume that you mean the expansion vessel. It does what it says, When you use your boiler, filled with water, Water expands. It needs somewhere to expand to. otherwise POP!. SO what is it filled with, Air, Before connecting it, IT has air in it, After connecting, Still has Air in it. BUT when pressure increases, Might get some water in it. Air will compress, water will not. SO when pressure increases, Due to the water in the system geting hot, and expanding, The Vessle absorbs the pressure increase due to the air in it. IF not for the vessle, Far to much pressure in the system, so leaks would appear all over, Plus at the maximum. Burst pipe on the heating system.
But basically the vessle is filled with air.
SD, November 2012
