Link your heating system to the 1/2 in. FNPT connection A . Connect directly a heating expansion tank (HGT-15, HGT-30, HGT-60M and HGT-60), with the Service valve (HGSV12) on the 1/2 in. FNPT connection B. Then, on connections C (1/8 in. FNPT), connect an automatic air vent (R8818) or a pressure gage (#GAGE0-100BOTTOM or #GAGE0-30BOTTOM). On connection D (1/2 in. MNPT) you can either connect your GMP or simply the cold water supply.