Eric, Typically you would install one air separator and that would be located on the supply out of the boiler and on the suction side of the pump. You will want to varify but looking at the Baxi Luna boiler the internal pump appears to be on the return side pumping through the boiler. If you are piping this primary/secondary, the air separator could be located on the secondary circuit, supply before the pump. You could locate a second air vent at a high point in the system for additional air removal.
Eric, Typically you would install one air separator and that would be located on the supply out of the boiler and on the suction side of the pump. You will want to varify but looking at the Baxi Luna boiler the internal pump appears to be on the return side pumping through the boiler. If you are piping this primary/secondary, the air separator could be located on the secondary circuit, supply before the pump. You could locate a second air vent at a high point in the system for additional air removal.