Can I Split Airflow From A 640cfm Hood, 10" Round To Two 3.25x14 Ducts - HVAC - DIY Chatroom Home Improvement Forum

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03-23-2010, 10:43 AM   #1
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## Can I split airflow from a 640cfm hood, 10" round to two 3.25x14 ducts

Hi. We are trying to upgrade our range hood as part of a kitchen remodel. Our current system vents via one 3.25x14 rectangular duct, down through the wall behind the range, then transitions to a 6" round under the house.

My range wall is a standard 4" depth, studs on 16", so 3.25x14 seems to be the largest rectangular duct the wall can facilitate.

The new hood is more powerful - 640cfm - requiring a minimum 8" round or equivalent. According to this website (http://www.engineeringtoolbox.com/eq...ter-d_205.html) the equivalent rectangular duct would need to be larger than 3.25x14; larger than my interior space between the studs and drywall.

So my question is, can I run TWO 3.25x14 rectangular ducts parallel to eachother in the wall (stud separating them) to meet minimum airflow? I realize I would have to join them at the top somehow, and not sure how to calculate the resistance of splitting the airflow like that. (The new hood has a 10" round vertical exhaust port at top, so I must make a 180 degree transition from that, to the two 3.25x14 rectangular ducts). Once under the house, the two rectangulars would rejoin/transition to a single 8-10" round to run out of the house.

Any tips are much appreciated. Jay

03-23-2010, 12:51 PM   #2
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If you have "standard" dimensional lumber, then the cross sectional area of a single stud space should be equal to an 8" round duct.

An 8" round duct has a cross sectional area of about 50.25 sq.in. (4*4*PI).

2x4 dimensional lumber is actualy 3.5"x1.5". So on 16" centers, you've got 14.5" or a total area of 3.5 x 14.5 = 50.75 sq.in.

Since you can build metal ducts to any dimensions, if you don't have to have the duct insulated, you should be able to build a metal duct the full 3.5 x 14.5 size.

Or is there something else I'm missing?

03-23-2010, 04:52 PM   #3

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Quote:
 Originally Posted by HooKooDooKu 2x4 dimensional lumber is actualy 3.5"x1.5". So on 16" centers, you've got 14.5" or a total area of 3.5 x 14.5 = 50.75 sq.in. Since you can build metal ducts to any dimensions, if you don't have to have the duct insulated, you should be able to build a metal duct the full 3.5 x 14.5 size. Or is there something else I'm missing?
You mean besides a 3¼ X 14½ is barely able to move as much air as a 7" round/oval duct.

 03-23-2010, 06:31 PM #4 Member   Join Date: Nov 2009 Location: central virginia mountains Posts: 1,857 Rewards Points: 1,000 if you hve to transition at the stove eliminate the stud, install header if bearing wall and have 30.5x3.5 __________________ The older I get the better I was
 03-23-2010, 06:32 PM #5 Newbie   Join Date: Mar 2010 Posts: 2 Rewards Points: 10 I wouldn't argue with your wall spacing, but HooKoo, your equivalent airflow specs seem to conflict with specs from the calculator results obtained here: http://www.engineeringtoolbox.com/eq...ter-d_205.html That calcs the 3.5x14.5 to only 7.35 round capacity at best. ???
03-23-2010, 11:24 PM   #6
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Quote:
 Originally Posted by ductdawgz I wouldn't argue with your wall spacing, but HooKoo, your equivalent airflow specs seem to conflict with specs from the calculator results obtained here: http://www.engineeringtoolbox.com/eq...ter-d_205.html That calcs the 3.5x14.5 to only 7.35 round capacity at best. ???
One of the things I wanted to point out was that you were using measurements of 3.25 and 14.25... I couldn't understand why?

Of course, like I said, my numbers were based only on cross sectional area... the page you're referencing takes into account the flow difference between a round duct and a rectangular duct.

Now I don't know if there are any code conserns in spliting the vent into two vents, but if a single stub cavity is equal to a better than a 7" round duct, any losses associated with splitting the flow is more than compensated by using the equivilent of TWO 7" round vents to vent a single 8" round vent.

 03-24-2010, 04:54 AM #7 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 34,584 Rewards Points: 8,154 OP. How much air do you WANT it to remove/vent. Start with that. And then see what size duct you need to use. Look at your hoods CFM rating at different static pressures(And no, the FR on a ductulator is NOT static pressure). Then figure out where the make up air will come from that the exhaust hood needs. For 50CFM of 10° colder air your make up air brings in. You add 540BTUs per hour of run time. EG: 70°F indoor temp, 40°F outdoor temp. 500CFM of make up air 30°F cooler then indoor temp=16,200BTUs of heat to warm it up to room temp(1 hour of hood exhaust run time).
 03-24-2010, 08:26 AM #8 Learning by Doing     Join Date: Jan 2008 Location: Easton, Maryland Posts: 3,156 Rewards Points: 2,000 Blog Entries: 7 Beenthere makes a good point. I can always tell if I forgot to turn off my exhaust fan because you can feel the breeze in my drafty house several rooms away. __________________ If I could only remember to THINK about what I was doing before I did it.
 03-25-2010, 12:47 PM #9 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 34,584 Rewards Points: 8,154 Just think what would happen if your house was tight.
03-25-2010, 02:35 PM   #10
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Quote:
 Originally Posted by beenthere Just think what would happen if your house was tight.
What would happen?
- pull exhaust from furnace into the house?
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If I could only remember to THINK about what I was doing before I did it.

03-25-2010, 03:16 PM   #11

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From the furnace and or water heater.
On a standard gas water heater it can also pull the burners flame out of the combustion chamber and burn up the water heaters insulation. And cause a fire. Possible on some older furnaces also.

Could also over power bath fan, and draw fumes from it.

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