Bob, An auto air vent can be placed on the supply or return side of the system and typically installed at a high point in the system. As long as there is positive system pressure it will not allow air back in. If it is at a point where there is a negative system pressure then it could draw air back in through the vent. Caleffi does offer an "Anti-Suction" cap for air vents that end up in that location under part number 562100.
Bob, An auto air vent can be placed on the supply or return side of the system and typically installed at a high point in the system. As long as there is positive system pressure it will not allow air back in. If it is at a point where there is a negative system pressure then it could draw air back in through the vent. Caleffi does offer an "Anti-Suction" cap for air vents that end up in that location under part number 562100.