Our house is a 1000 sq foot ranch with a 500 ft Family Room Addition built on a slab. to get supply heat and a/c into the room, I had to run an 8"supply duct in my crawl space upt to my attic, a distance of 25 feet. Off of that I have 4- 6x10" vents on the perimeter walls that are fed by a 6" supply. Problem #1- My first two vents that are fed by 2- 6" 90Degree elbows get very little flow. they are not the end of the run but in the middle. Problem #2- I have trouble keeping the room warm even with the new windows and the attic insulated. Average Temperature inside is 62 degrees when outside air temp is 20 degrees. I tried a duct booster and it doesn't solve the problem doesn't help at all. Trane Furnace XE80, 90,000 BTU Downflow.