I posted the numbers. You got me - I do not know what you are refering to with the return side unless it is the return air to the blower? In which case it is simple. The air handler is in an enclosed closet from floor to ceiling - approx. 10 feet. there is one louvered opening to this closet approx. 3 feet square. Also a motorized vent to the out side through the roof - this because the house is too tight for health full air exchange , so it opens every time the blower comes on and closes when it is off. All of this is in a utility room eight feet wide and fourteen feet long - one door to outside and one door to the interior of the house. The inside door is normally open and air flow will force it open even though it has a sizable louvered opening in it. The blower does change speeds. the unit has a logic board with dip switches - these were reset the first time I had to call for repairs. This was done with phone consultation to the supplier. Thanks, THPRE