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Old 02-22-2010, 07:54 PM   #1
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Are the duct sizes wrong also?

My furnace is in the basement with a reducing plenum duct design. If I have that bit of terminology correct, I 'd like to see if the trunks and branches were designed correctly. I'm sure I'm way off, but should the cross section area of the branches be less than or equal to the area of the trunk?

For example, the far end of the supply trunk measures 8x14= 112 sq. inches. The four 6" round branch area is also 112 sq. inches. The next trunk step larger measures 20x8 for 160 sq. inches, but has eight 6" round branches that total 224 sq. inches. The trunk with the a/c evaporator measure 20x20 I believe. So, is the 112 sq." measurements a coincidence, or is the first trunk too small for the number of supply branches?

I wasn’t sure if the trunk size and number of branches were equally proportionate, or purposely undersized to create pressure. Thanks again.


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Old 02-22-2010, 09:58 PM   #2
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A 20X8 duct moves more air then 8-6" round ducts do at the same FR.

A 10' long section of 20X8 duct has a total surface area of 46.66 sq ft.
8-10" long sections of 6" round duct has 124.8 sq ft of surface area. So they would be far more resistive to air flow. And move less air.


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