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Old 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

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Old 03-23-2010, 12:51 PM   #2
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Can I split airflow from a 640cfm hood, 10" round to two 3.25x14 ducts


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?

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Old 03-23-2010, 04:52 PM   #3
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Can I split airflow from a 640cfm hood, 10" round to two 3.25x14 ducts


Quote:
Originally Posted by HooKooDooKu View Post

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 3 X 14 is barely able to move as much air as a 7" round/oval duct.
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Old 03-23-2010, 06:31 PM   #4
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Can I split airflow from a 640cfm hood, 10" round to two 3.25x14 ducts


if you hve to transition at the stove eliminate the stud, install header if bearing wall and have 30.5x3.5
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Old 03-23-2010, 06:32 PM   #5
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Can I split airflow from a 640cfm hood, 10" round to two 3.25x14 ducts


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.

???
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Old 03-23-2010, 11:24 PM   #6
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Can I split airflow from a 640cfm hood, 10" round to two 3.25x14 ducts


Quote:
Originally Posted by ductdawgz View Post
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.
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Old 03-24-2010, 04:54 AM   #7
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Can I split airflow from a 640cfm hood, 10" round to two 3.25x14 ducts


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: 70F indoor temp, 40F outdoor temp.

500CFM of make up air 30F cooler then indoor temp=16,200BTUs of heat to warm it up to room temp(1 hour of hood exhaust run time).
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Old 03-24-2010, 08:26 AM   #8
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Can I split airflow from a 640cfm hood, 10" round to two 3.25x14 ducts


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.
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Old 03-25-2010, 12:47 PM   #9
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Can I split airflow from a 640cfm hood, 10" round to two 3.25x14 ducts


Just think what would happen if your house was tight.
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Old 03-25-2010, 02:35 PM   #10
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Can I split airflow from a 640cfm hood, 10" round to two 3.25x14 ducts


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Originally Posted by beenthere View Post
Just think what would happen if your house was tight.
What would happen?
- pull exhaust from furnace into the house?
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Old 03-25-2010, 03:16 PM   #11
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Can I split airflow from a 640cfm hood, 10" round to two 3.25x14 ducts


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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