Connect 8 inch flex duct to 6 inch fitting?

[goshenHeat]

Ranch house AC system - air handler + duct work - is in my attic with ceiling vents in the house. The main run is square rigid duct (~ 3' x 3') with fittings off the rigid duct to which round insulated 6 inch flex ducts are attached. The flex ducts connect to the ceiling AC vents. Most flex runs are less than 10 feet. However one 6 inch flex is 25 feet long. I do not get much air flow out of this duct. I want to remove the 6 inch flex and install 8 inch flex since an 8 inch flex has roughly twice the flow of 6 inch at the same pressure. (Yes I know that rigid is best for a long run but other considerations prevent me from using rigid.) The fitting coming off the main rigid duct is a round 6 inch fitting.

(1) If I connect the 8 inch flex to a 6 inch fitting, will I get more air flow or will the 6 inch fitting restrict the flow so that using 8 inch flex will produce no additional flow?

(2) How do I connect the 8 inch flex to the 6 inch fitting? Can I crimp the 8 inch flex to the 6 inch fitting, or do I need a 8" x 6" reducer?

(3) Is the best approach to remove the existing 6 inch fitting and replace it with an 8 inch fitting?

[HayZee518]

6 to 8

use either a left hand cutting or right hand cutting tin snips and increase the size of the outlet to 8 inch. the 6 to 8" fitting will act as a venturi restricting the flow. then just use an eight inch takeoff fitting to your flex duct.