The function of a MonoFlo tee is in its construction. It can lie in any orientation. In fact, they point downwards for basement radiators. As far as air is concerned, it should be evacuated at the boiler, and NOT get to the radiation, but if it should it will still rise to the highest point in the system. I assume they gave you the parameters for the distance between the MonoFlo tee and the "return" tee from the radiator, and if you have any basement radiators, you need TWO MonoFlo tees.