Hey Dom this what I have…….Removed and Replaced a weil-mclain CGA 70/50 system did a pressure test now looking to fill the system…the only spicket is the factory drain valve on the single zone system 3 cast iron baseboard’s on the first floor with screwdriver type bleeders and four old fashion stand-up with the sq. bleeders on second floor… My game plan is to open the water and bleed the first floor one at a time then do the same to the second if this is proper at some point do I have to open the drain valve to get water in to the boiler it-self and should the automatic air vent be open or close during the process… ???