A forced warm air duct heating/cooling system should have a Return "area" that is at the LEAST equal to the Supply area.
In other words, if the Main Supply duct is 8x20 (160 sq in ) then the Return size should be at least 160 sq in or even approx 10 % more.
Hopefully, the Supply caculation and the Unit (btu and cooling tonnage) are sized properly in the first place via Manual J and Manual D to determine the correct heat loss/gain.
As for placement of Returns,they are best on the floor opposite and farthest away from the Supplys which are best when placed on the exterior of the rooms (under windows)
Ex; Supplys on the exterior of the room
Returns on the interior of the room
note; for cooling purposes only, Returns can be placed in the interior high-wall location, or for both heating and cooling they can be located in the SAME stud space and activated by installing a damper in the lower grille.
Last edited by Hube; 08-20-2008 at 08:02 AM.