Duct Sizing Question

If I'm using 8" round duct to facilitate my returns, shouldn't the register boots and the takeoffs have the same area as the 8" pipe?
I have the same question with the supply boots as well. If the area of 8" round is 100 square inches, why then are there so many boots with dimensions like:
4 x 10, 6 x 10, 8 x 10 ?
It appears none of these boot openings or takeoffs have adequate cross-sectional areas to facilitate the round duct size I'm running to them. Won't the air volume be limited by the smallest piece in the branch?
I do realize this question is probably stupid, but my brain short circuited on this for some reason.
Thanks in advance for any help.
Richard
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were is you air duct calculator? 8" rd duct only has a 220 cfm capability with .10 static so, (2) 6" returns will be the max or you will get whistling

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8" round is about 50 sq in, not 100. The max velocity you want in a return run in residential is 700fpm. We try to keep face velocity at the grille under 500fpm.
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please explain how you go about doing that when a variable speed furnace blower will vary its delivered cfm from 50 to 100%. :)

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You're a PITA, Fish. :) But to answer your question you design for the max air flow, but then you knew that.
IMO, the trick really comes into play when you have AC with floor registers and a variable speed system. As far as I can tell, you're going to have pooling. Good case for radiant floor heat and AC with ceiling registers.
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The duct has an air flow resistance related to it's size and length. It may be legitimate to have a bigger cross section on a long run to improve the air flow even though there is a lower cross section at the end.

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Richard wrote:

The linked pages below may not be perfect, but might help answer some individuals' questions. - udarrell
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PROPER A/C UNIT & DUCTWORK Sizing is over 70% of Operating Efficiency
http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html
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