Added 2 elbows to ductwork - airflow issue

[Matthew512]

We are finishing an unfinished basement and learning as we go. We did not want our ductwork in the bathroom we are adding, so i added 2 elbows to the bedroom run and now i have less airflow in the room. I did not know that adding elbows would reduce the airflow as much as it did.

What i have:
a 20 foot run with six 90 degree elbows, plus the final elbow that delivers the air to the upstaires bedroom. It's all piped with 6" solid pipe. Previous to my addition there was 4 elbows plus the final one up.

Question:
If i redo the pipe to a bigger diameter will that fix the issue? how much bigger would i need to go? 7" or 8"? i have plenty of space for a bigger pipe.

Maybe the current 6" pipe is fine and i should just play with the dampers to help get even airflow throughout the house? i'm not thinking this is the way to go though.

Furnace: Ducane model #cmpe075u3b.

Thanks for the advice and thank's for all the other useful info on the site!

 

[Jadnashua]

Long curves work better than sharp bends, and bigger ducts can provide more airflow...there are some calculators out there that can help you determine. A sharp bend creates turbulence and increases backflow, both of which decrease effective air flow through the duct. Smooth out the bends if you can, and it may be enough.

 

[Sponsor]

 

[Dana]

What he said. Sharp throated ells are like adding 40-50 "equivalent feet" of duct or more, radiused ells add something around than half that. Radiused ells with internal turning vanes do even better. For a round ducts with segmented 90 degree ell, no turning vanes it's ~30 equivalent feet per ell:

Fatter duct is the only way around the higher duct impedance when you've introduced some twists & turns. Short or running a full Manual-D duct re-design, if you have the space, go with 8" and install a tweakable duct damper to dial it back.

Be sure to seal every seam & joint with duct mastic too, and caulk the duct boots to the wallboard or floor.