HVAC ductwork sizing

[bmwpower]

Ok....So I'm redoing the ductwork in this place my parents bought. The old stuff is the kind with the insulation on the inside. The previous owners smoked and had pets and the inside of the ductwork is just $%%^&&*.

The old ductwork (main trunk in the attic) looks like the attached picture from above:

Probably about 24 feet in length with 8 - 6" circular trunks coming off both sides. The HVAC unit ductwork enters from below about the center of the main run.

I want to replace everything...

Why is the ductwork shaped the way it is? Why the decrease/increase in size? Why not the same size all the way across? How does this size change affect the air flow at the registers?

Can I buy this replacement ductwork online?

[bmwpower]

Where's Mr. HVAC?

[mleichtle]

I can be that guy, I took a class.

Friction, downstream from branches, turns, and even whatever distance of strait run, you have to downsize the duct to keep the cfms constant to the end of the run.

[MXtras]

I am not Mr. HVAC but I did stay at Holiday Inn Express a few weeks ago...

The shape is to maintain a somewhat constant plenumn pressure down the length while the volume is being reduced. Without the shape, the pipes at the end would deliver more volume than those located upstream unless the diameter of the pipes at the end was reduced or otherwise reduced and that could create higher velocity and might create whistling over sharp edges. The smaller section of the plenumn creates restriction to force the air to flow where it otherwise would not want to.

That's my story and I am sticking to it.

Scott

[imported_banzaitoyota]

The Ductulator is your friend!