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  1. #1

    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 ( 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

  2. #2
    Join Date
    Jul 2008
    Rochester NY
    Oh boy. Do you have any natural draft appliances? Old fashioned water heater?

    Be very careful depressurizing your house by 640 cfm, or at the very least remove flammables from the vicinity of your water heater!

    I'm not an airflow expert but that approach sounds ok for the fan airflow. Someone more knowledgeable about that will probably share their 2c.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to, or RESNET, and find an auditor near you.

  3. #3
    Yeah, she's got some power. Our natural gas water heater is in the garage, outside of the interior vacuum.

  4. #4
    Join Date
    Feb 2010
    Whatever you decide to do, test (smoke , noise, etc.) run the hood prior to concealing.

  5. #5
    Join Date
    Oct 2009
    Raleigh, NC
    You should have lots of options to choose from. If it were me, and the exhaust run under the house is not too long, I would just use the one duct. 640 CFM is twice as much as you should ever need for range venting. If there is a restriction by the too-small duct, it won't be that bad. It shouldn't hurt the fan motor at all. It is either shaded-pole or PSC. There are probably other options, like, is this a one-story house? If it is, I might consider re-routing through the roof. This way you could use 8-inch round straight up. There would be far less restriction compared to the rectangular ducting and all the transitions and elbows that you are intending to do with your current setup. If it is an exterior wall you could vent straight through it, of course. On a related note, I removed and replaced a Kitchen-Aid range hood three weeks ago. It had twin squirrel-cage blowers and a very large rectangular outlet. Whomever installed it made a sloppy transition to FOUR INCH VINYL DUCT...The same kind you vent your dryer with. It had been this way for over 15 years, according to the homeowner. She had no complaints about the old one, but she thinks the new one is so much better (even though it is probably a lot lower CFM, it is just vented properly now).

  6. #6
    Join Date
    Jan 2004
    Ain't gonna be moving no 640CFM with that set up. Which is ok. Since you don't want to pay to condition 640CFM of make up air.
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