The boiler has an intergrated expansion vessel (Big red spaceship shaped thing on back) This has a diaphragm in it separating the water and air side.You have no air on the air side. Take off the valve cover (Like a bike tyre valve) and push in the valve pin. If water comes out the expansion vessel is knackered if air comes out you need to repressurise it. Let out system pessure and use foot pump to pump up to .75 bar. refill system and test.
Gas Man, November 2009
