I have a 3 1/2 ton and a 3 ton unit on the ground but I have two 4 ton units in my attic. I live in Arizona. I just had an energy audit done and they say I need two 20 x 40 returns with 20" ducts running back to the unit. Currently I have a 20 x 40 return with a 18" duct going back to the ac unit and a 20 x 30 with a 16" duct going back to the unit.