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Old 09-09-2012, 08:47 AM   #1
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Flatter duct work


There is a branch (7" x 11'") off the main supply that is surface mounted to my joists.

Since I am trying to gain some extra head clearance, I am wondering if I can replace the existing duct with a flatter profile duct (say 4" x 20")?

Theoretically I would be moving about the same volume of air?

What do you think?
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Old 09-09-2012, 09:00 AM   #2
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Flatter duct work


Yes you can, if you go shorter you just need to make up the volume by widening the duct work.... gets a little more expensive, but will give you the head room.
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Old 09-09-2012, 10:19 AM   #3
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Flatter duct work


awesome. thank you
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Old 09-09-2012, 12:01 PM   #4
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Flatter duct work


Air starts acting goofy when you get out past a 3/1 aspect ratio. It tumbles in a more circular motion not using the corners of your 7x11. Dropping down to a 4x20 will have 2 columns of air with neither using corners. So even though the area is the same it will not have the same air flow. Still could be good enough if the room really didn't need that much to begin with.
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Old 09-09-2012, 12:33 PM   #5
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Flatter duct work


Its not just the open area of the duct that counts. Its the amount of surface area also. A 7X11 duct has a 3 sq ft of surface area for every 1 foot of linear length, and a 4X20 has 4 sq ft of surface area for every 1 foot of linear length. So 4X20 won't flow as much air as a 7X11 at the same static pressure. You would need to be about a 4X24, and the take offs would have to be set back almost 6" from the face of the duct, to get the same amount of air to go into the take off.
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Old 09-09-2012, 03:37 PM   #6
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Flatter duct work


thanks for the lesson in air flow!

i decided to keep it as is, but extend it towards a wall.
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Old 09-09-2012, 04:40 PM   #7
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Flatter duct work


Your welcome. keep us updated.
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