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Old 03-11-2011, 09:54 AM   #1
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Duct size


I am supplying duct in a 17x22 room with a 8x12 trunk. I need to pull two lines off the trunk for vents. I am wondering if two 7" lines to two 4x10 vents will be enough.
Any suggestions would be appreciated. I have 8" lines lying around also if that would be better.
Thanks in advance DJ

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Old 03-11-2011, 11:33 AM   #2
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Duct size


17x22x.9 = 337 cfm
12x8 metal rectangular duct = 400 cfm
7" round metal duct = 125 cfm (110 in flex)
8" round metal duct = 185 cfm (160 in flex)
vents: 4x10 x2.5 (some say 2 some say 3) = 100 cfm
12x6 x2.5 = 180 cfm
so..I would say use two 8" with 12x6x8 boot

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Old 03-11-2011, 01:53 PM   #3
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Duct size


Poppyloppy: I note that your figures are derived using a .075 SP. Here in the south, for residential applications, we use a 0.1 SP for supply air (of course, we have a large heatload to overcome...). Just curious if that is the standard in your region and why? Also, if the OP is placing his supplies at one end of the room on the 17' wall, would this prompt you to increase your static pressure in the ductwork and increase velocity/throw via a smaller supply box opening/grill size?

Mark

Last edited by rightit; 03-11-2011 at 01:56 PM.
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Old 03-11-2011, 02:09 PM   #4
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Duct size


Well. a .025 SP difference shouldn't really matter. I believe most specs call for total system SP of .50 on single stage and nobody can get that and some can't even get close. Example a 5 ton wet coil alone can be .41-.47.

The extra push you're talking about is usually related to the length of the run. If over 25' move down 1 size, just make sure that the final duct size and grille size match the room cfm you're looking for
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Old 03-11-2011, 03:00 PM   #5
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Duct size


Thanks for the quick response. Ur a life saver. DJ
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Old 03-11-2011, 04:29 PM   #6
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Duct size


Quote:
Originally Posted by rightit View Post
Poppyloppy: I note that your figures are derived using a .075 SP. Here in the south, for residential applications, we use a 0.1 SP for supply air (of course, we have a large heatload to overcome...). Just curious if that is the standard in your region and why? Also, if the OP is placing his supplies at one end of the room on the 17' wall, would this prompt you to increase your static pressure in the ductwork and increase velocity/throw via a smaller supply box opening/grill size?

Mark
A duculator doesn't tell you what the static pressure will be. It tells you what the static pressure loss will be from the ducts surface friction.

There is just about no such thing as a 100 foot only TEL duct system. if you test your systems, you'll find your running a lot higher static then you think.

Just a FYI.
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Old 03-14-2011, 08:43 PM   #7
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Duct size


Thanks for the comments, PL and BT.

Mark

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