# Can 2 - 6" flex ducts be combined into a single 8" rigid duct

I'm trying to clean up some sloppy HVAC work in my home prior to finishing my basement. I have 2 VERY long 6" flex duct runs coming off the supply master that feeds the great room in my house. Both runs make a hard 90 degree turn at one point as well, which i understand is a no-no with flex-duct. Both ducts also run parallel to each other for most of the run.

What I'd like to do is combine the flex duct runs into a single, rigid duct up until the point where they would split. Then use a T or Y to attach the flex for the last few feet of each.

I've been reading about calculating the CFM and it's making my head spin. From what I've read, it seems that flex does have more friction than rigid. THe area of 2x6" circles is 56.5486. The area of a single 8" circle is 50.2655. So, it's relatively close in terms of area.

Would I be OK using an 8" rigid duct prior to the split, or would I need to go up to a 10" duct. Space is a consideration, as I may not have room for the 10". THANKS!

• You're probably OK with reduced area because of less friction, especially if the sharp bends are eliminated. I'd still rather be safe and go bigger. If 10" round doesn't fit you could always go with rectangular ducts. Feb 25, 2014 at 21:29
• The ACCA Manual D equates 6" flexible to 5" rigid. The areas of these circles: 5" dia -> 19.6 sq inches, 7" dia -> 38.5 sq in; 8" dia -> 50.3 sq in; 10" dia -> 78.5. Based on area alone, I think 8" would be sufficient for you. If you do the calculations for friction and CFM, you might get by with 7". I think 10" would be too large. Jul 7, 2016 at 20:32