I have a 2.5 ton AC on an 80,000 BTU gas furnace. The existing (single) return consists of a 20 x 20 filter grille and approximately 20 feet of 14" round pipe with two 90 degree elbows. This connects to a return plenum under the furnace.
I know I should increase the filter size to either 20 x 24 or 20 x 25 (the 20" width can't be increased, but I can accommodate some extra height). Is the 14" pipe large enough for this installation?