I guess I have a combination issue. The return is undersized so the unit only works right, if door to closet the unit is in, is left open. Original owner cut hole in second floor/closet ceiling, size of return duct 8x24, I added 8x12 through wall above door so I am still 152 sq. in. short. I guess if you use size of unit to determine the proper size. 20 x 22.it may be expensive, but you don't need to replace it every month!