Duct Diameter Reduction

Earnie

Is there a real need to reduce rigid duct diameter after branches tap off a trunk?

I am at my next phase of replacing flex branches with rigid pipe. I currently have a 12" rigid trunk that runs 20ft with three 8" and one 6" flex branches. As the branches tap off the trunk, it continues at 12" diameter and terminates three feet after the last 8" branch.

I'm sure keeping ample FPM is the concern. Should the 12" trunk reduce in diameter to 10" after the first two branches? Or is the trunk run too short to make a difference?

Earnie

Anyone?

sktn77a

Well, as your takeoffs progressively "steal" air from the supply trunk, there's less volume (and therefore less velocity) per unit time. This could reduce air "throw" from the register so that air distribution throughout the room is compromised. Usually though, you have the opposite problem - too much velocity from undersized ducts and oversized equipment. If your current airflow/distribution is fine, I'd leave the trunk alone. You may need to reduce the branch diameters if you go from flex to hard pipe - the resistance to flow for flexduct is about 25% higher than metal ducts.

beenthere

Under 24 foot in length, you don't need to reduce the trunk line.

Earnie

Thanks guys. Will make it a bit easier by re-using the existing 12" pipe.