I was hoping I could get some opinions on a recommendation made by my contractor. The house is less than a year old and we have found many problems with the ductwork, mainly with the return being severely undersized. The contractor has fixed the returns by adding two more 30 x 8 return grilles and by running one 12 and one 10 flex duct from the new grilles back to the return box which is mounted on the side of the furnace in the attic. According to him I now have a total of 2.9 sqft of return duct area going back to the furnace. Here is my equipment information (Goodman): The furnace is model GMT090-4B, the condensing unit is a CKL42-1B (3.5 ton), and the coil is a CACF0422B2A (3.5 ton). All ductwork is flex.
My question is regarding return filter area. I currently have one 25 x 16 x 1 in the return filter box thats mounted on the side of the furnace. Thats only 400 sqin of filter area and Goodman recommends a min. of 770 sqin. Originally the contractor wanted to put another return box on the other side of the furnace with another 25 x 16 filter, but because of space limitations it would be tough. So he recommends installing a 5 pleaded filter between the side of the furnace and the current return box. According to the literature he showed me the 5 filter has more than enough filter area (78 sqft). To me the 5 filter sounds like the way to go. What to you guys think? Based on the area of return duct I now have and the proposed 5 filter would this mean all return components are sized correctly now?