I am working on an install of a 2 ton A/C with a upflow furnace. The top furnace opening is 12" x 18". But the coils are 12" x 23". The start of my main trunk is 12" x 18". Can I transition the plenum right off the coils to a 12" x 18" over the 5" area that's dead air? Or will doing that cause me have air restriction?
Here's my plan: I would like to transition the plenum from 12 x 23 right off the coils back to a 12 x 18 and rise 34" and 90 into the attic where it will meet the main trunk which is 12 x 18. I am afraid if I leave the plenum at 12 x 23 and make the transition to 12 x 18 after the 90 that I will lose a lot of air velocity. Should I make that transition off the coils?