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DIesel Jay 03-11-2011 09:54 AM

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

Poppyloppy 03-11-2011 11:33 AM

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

rightit 03-11-2011 01:53 PM

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

Poppyloppy 03-11-2011 02:09 PM

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

DIesel Jay 03-11-2011 03:00 PM

Thanks for the quick response. Ur a life saver. DJ

beenthere 03-11-2011 04:29 PM

Quote:

Originally Posted by rightit (Post 607362)
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.

rightit 03-14-2011 08:43 PM

Thanks for the comments, PL and BT.

Mark


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