What is the best location for installing the humidifier? What is the direction of the air flow through the humidifier for a model 400 aprilaire humidifier. It has a cross connect between the supply and the return. Some installation instructions state the supply plenum where other instructions state the return. I believe the air flow is from the supply plenum to through the humidifier to the return. If this is the case, it seems that the return plenum would be the best location. Need your help. Thanks!