I have a 3.5" x 12" supply duct running from my basement up to my 2nd floor. The supply pipe from the furnace to the rectangle duct is a 6" round and also from the end of the rectangle duct to the register. The heat just trickles out upstairs. I was wondering if changing the 6" round to an 8" round off of the furnace would make much of a difference or can someone recomend a different size to get addequate airflow to the 2nd floor? I tried a booster fan and that really didnt make much difference. Any help is appreciated.