We recently added on to our house and have a question on the heat ducts. We have forced air heat with duct work running in the crawl space. The duct is 8"x24" and approx.20'long. There are about 8-6"round ducts going to each room, branching off the main. We added a new duct, 8"x20"x16'long off the main about 4' from the end. We tapped into the side of the main and go straight out to the addition where we run 4- 6" round runs to rooms in the addition. We also tapped into the cold air return and added 1-6" & 2-8" returns. Our furnace is a 3 yrs old Bryant 80 and should heat 2000 sq ft. Our house is now 1600 sq ft. The question is, why doesn't the new heat ducts have much air flow? Is it where we tapped into the main or do we need a blower added to the duct? Any suggestions would be appreciated.