Valillant Boiler VCW GB 242E ?

Keeps loosing pressure when pressurised to1.5 bar ,hot water work fine,but heating side. I have replaced new prssure relieve valve. As fill 1.5 bar when heating working pressure goes up,I believe it is Air pressure vessel to needs to be pessurised ,but dont know how to fill and is 10 p.s.i. where is filling point.i will appreiate if someone answer my question thanks for help.