Brian, The smaller brass air separators are ok to pipe the expansion below the unit. The larger steel models we do not recommend using that location to pipe the expansion tank too. If you have a really dirty system than I would not recommend that location for the expansion tank because the air separator could remove dirt into the expansion tank. The idea with the expansion tank and fill located below is because it is installed at a point of no pressure in the system.
Brian, The smaller brass air separators are ok to pipe the expansion below the unit. The larger steel models we do not recommend using that location to pipe the expansion tank too. If you have a really dirty system than I would not recommend that location for the expansion tank because the air separator could remove dirt into the expansion tank. The idea with the expansion tank and fill located below is because it is installed at a point of no pressure in the system.