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Purchased a Kitchenaid Range Hood Vent that has a 1200 CFM blower. Instructions say to install a 10" duct vent. My existing venting duct was a rigid but small 6 inch duct.

Unfortunately at demo realized that sewer line and other utilities are right in the middle of the ceiling cavity perpendicular to the exit point, 10 feet away = cannot move (cost prohibitive).

A. Can I used TWO 5" rigid ducts instead of the 10" one required? How? I'd think the biggest challenge will be the multiple reducers needed to go from 10-8, 8-6, for both lines. And a Y-joint somewhere...

B. What if I used the existing 6" duct and merely added a 4 inch line? Or two-6 inch ducts?

(I will also be adding some mineral wool insulation for sound absorption right above the range but the goal is not to have to tear down the ceiling either.)

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    Cross sectional area of a 5" duct = 78.54 square inches. Cross sectional area of a 10" duct = 314.16 square inches... 6" duct = 113.1 square inches, 4" = 50.27 square inches... – Comintern Sep 18 '16 at 19:00
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    That is a lot of CFM, is your cooktop huge or kitchen massive? You may not need that much CFM – Jack Sep 18 '16 at 19:16
  • In other words, 2x 5" ducts gives you a 157.08 square inch duct (cross section), which is a lot less than the 314.16 square inches you get from one 10" duct. So if you actually REQUIRE the equivalent of a 10" round duct, this won't do it. You'd need almost 4x 5" ducts to do the job. If you were talking rectangular ducts, it would be a different story. – Craig Sep 18 '16 at 19:17
  • @Comintern's values are incorrect but the ratio between duct sizes is. Formula for area of a circle is pi * radius squared, not diameter squared. So 5" is about 20 sq in and 10" duct is about 78 sq in. – topshot Sep 18 '16 at 23:26
  • @topshot - You're right of course, I guess I should check what the inputs of online calculators are... – Comintern Sep 18 '16 at 23:28
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Here are the numbers and you can see what your options are.

The numbers are as follows:

On the left: CFM (Cubic Feet per Minute, cfm)

On the right: Equivalent Diameter Round Duct Sizes

200 : 4.9

300 : 5.7

400 : 6.6

750 : 8.3

1000 : 9.1

1250 : 9.8

1500 : 10.7

1750 : 11.5

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