There is a fault on your boiler expansion vessel.
either it has no air or diapharm is split.
Check the air filling point is on the top. it is like a tyre air filling valve. press it if water comesa out it is split if little bit of air comes out you need to fil air in it up to 10 psi
IF WATER COMES OUT YOU NEES TO REPLACE THE VESSEL.
HEATINGDOC
October 2007