I have removed a wall in the kitchen, the straight heating duct running through the wall from the basement to the second floor bedroom is 3in by 10 in. (30 sq in.) .I want to replace it with a round duct with 5 elbows that will run from the existing vent in the bedroom, down from it to under the floor, along the kitchen ceiling towards the back of the kitchen, down the back wall, under the kitchen floor to the existing heating duct, the original 3 by 10 duct was 8 feet long , to connect the bedroom to the basement will require 16 feet of duct with elbows .what size of duct should I use.

I think I need molre info before answering. Is this duct supplyiong airto another part of the house or just the kitchen. Why so many elbows, each elbow will reduce the air flow.