Kitchen Exhaust fan CFM expecation

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Old 12-04-19, 06:26 PM
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Kitchen Exhaust fan CFM expecation

My kitchen is on the first floor. There is one floor above kitchen then attic
Currently my over the range microwave (350 CFM) is not vented outside. I want to vent outside.
Since the inside walls are only 4' thickness running the 4' or 6' metal duct through 2nd floor through the attic will be tough i think.
So i am planning my next option which is running the duct 3-4 feet up from the microwave then 90 degree bent then 30 feet to the back of the house (between joists over the drywall ceiling)

What i am trying to figure out is how much of 350 CFM i will loose for:
(1) 34 feet length 4 inch round metal duct with one 90 degree bent?
(2) 34 feet length 6 inch round metal duct with one 90 degree bent?
(3) using flexible duct reduce the CFM much further?

Thanks!
 
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Old 12-04-19, 10:44 PM
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The 4" is way too small.
The 6" is better but the distance is going to kill you.
Flexible duct is out..... especially for a kitchen exhaust fan.

Why is the run so long ?
30' run to the back of the house..... is the kitchen in the front ?
 
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Old 12-05-19, 12:54 PM
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6in duct will only be good for about 100 cfm
 
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Old 12-05-19, 03:16 PM
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There are online calculators for figuring the loss. Most manufacturers websites also have guidance on duct length. I know of none that use 4" and 6" is used for the lowest powered vents. You don't even want to consider flex line as it's resistance is even higher.

Microwave vent hoods are notorious for poor performance (useless). The last thing you want is a long duct run with 90 bends. Normally a 90 adds about the same resistance as 5' of duct so you may be close or over the maximum recommended length. If you want the vent to actually be useful consider adding a duct booster fan.
 
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Old 12-08-19, 01:47 PM
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Thanks for the comments Pjmax, airman 1994, Pilot Dane.
Kitchen is behind the garage in the middle. Its a town home. duct has to cross kitchen length And living room (30 feet).
can't vent it to the front (by code) and side ( have to cut joists).
I will consider adding a duct booster fan.


will measure going up throught the 2nd floor through and roof and see if the distance is less.
 
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