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Reducing a range hood vent from 6" to 4"

Scottybm
Scottybm Member Posts: 1
edited June 29 in Chimneys & Flues

Hello I have bought a 30" 380 CFM Cosmo range hood and it will be placed on a outside wall. The exhaust collar on the top of the hood is 6". The ducting will only have to travel up 12 to 14" and then a 90 Degree elbow to the outside, can I reduce to a 4" duct going outside?

Comments

  • Ironman
    Ironman Member Posts: 7,537

    Not according to code. A 4” pipe only carries 1/3 of what a 6” does.

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 9,258

    Depends on how powerful the fan motor is. One of two things will happen,

    1. The air must move faster thru the smaller duct
    2. Less air will move at the same velocity thru the smaller duct

    My guess is there will be a combination of both. The 380 CFM rating is based on the static pressure per 100 ft of duct. Since you do not have 100 feet of duct, then the static pressure will be much less than the rated capacity.

    Here are some calculations to review

    Screenshot 2024-06-29 at 11.24.12 AM.png

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • HomerJSmith
    HomerJSmith Member Posts: 2,628
    edited June 29

    Of course you can. Nobody is holding a gun to your head except maybe Mother Nature. It's not something I would do. The outlet of the appliance determines the minimum duct size after that the length of the duct.

    You came to this site to find out the right way to do it.

  • Zman
    Zman Member Posts: 7,611
    edited June 29

    Mid-grade hoods like that usually overstate their CFM capacity and rarely provide the fan curve you would need to figure this out. You will reduce the capacity significantly if you reduce it to 4". I generally increase the duct one size on these unless the run is super short. Another question you might ask yourself is where the 380 CFM will come from if the fan will really move that much? That's a decent amount of air and you don't want to cause a backdraft at other vented appliances.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • pecmsg
    pecmsg Member Posts: 5,271

    Look into a real exhaust fan.

    Fan Tech!

  • Kickstand55
    Kickstand55 Member Posts: 112

    Don't forget makeup air! Once the building atmospheric pressure heads to zero, air movement slows as do cfm's.

  • ethicalpaul
    ethicalpaul Member Posts: 6,520
    https://forum.heatinghelp.com/discussion/comment/1805063#Comment_1805063

    What is the code? I see a code that says that 100 cubic feet/min is required. This fan is 380 so even at 1/3 of that it might be compliant. I'm asking for my own education and am not arguing, thank you.

    NJ Steam Homeowner.
    Free NJ and remote steam advice: https://heatinghelp.com/find-a-contractor/detail/new-jersey-steam-help/
    See my sight glass boiler videos: https://bit.ly/3sZW1el

  • Jamie Hall
    Jamie Hall Member Posts: 24,693

    The fan is 380 — but at very low pressure. Do you have any inf9ormation as to how much back pressure the smaller ducting might cause, and how that will affect the fan performance?

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,576

    Hi @Scottybm , Backing up a bit, my question is about why you want to go to a 4" duct. Is there limited space to run the duct? If so, you may be able to use rectangular duct that does not restrict flow, but still fits where it needs to. Can you give us more info on what you're facing? Thanks.

    Yours, Larry

  • jimna01
    jimna01 Member Posts: 39

    My kitchen ycontractor (who otherwise did a great job) installed a 4 inch duct for my 380 cfm vent. The fan motor burned out in less than a year. I replaced the duct with the a 7 inch as specified by the manufacturer. The 4 inch duct wouldn't allow the fan to removed the smoke from stir frying or pan searing a steak either. You also have a pretty long duct run and a 90 degree. Go with the 6 inch duct. The 4 inch will significantly hurt the performance.

  • PRR
    PRR Member Posts: 226

    Aside from logic and experience: RTFM!! Read The Factory Manual!

    Cosmo is the marketing name for many products. I picked, at random, a "30 inch 380CFM" job. COS-63175 / 63175S manual from Home Depot site: https://images.thdstatic.com/catalog/pdfImages/1c/1ccb9721-8634-4a27-9b40-e5f741178b33.pdf

    Cosmo-Venting.gif

    Page 14 has multiple restrictions. Insurance and Inspection want you to meet "all" factory requirements. 4-inch caps are banned. Backdraft!!

    IIRC, the basic $49 fan for cheap bath or kitchen is 70CFM. IMHO this is hardly enough for a fart, and won't keep up with a busy cook's smoke and steam. That's why you got 380CFM. Which BTW is near half the size of my whole-house heat furnace (850CFM). It needs substantial ducting!

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