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Thread: Range hood pipe size

  1. #1
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    Range hood pipe size

    Have a job later this week to install a vent pipe for a range hood. It started as venting the existing microwave and I was going to run 6" pipe. Now they are shopping for a ~600 cfm hood. A 900 cfm hood I found is calling for an 8" pipe so by my guesstimation a 6" should handle the 600 cfm unit.

    But no one online is listing the velocity of the blowers so I'm a little stuck trying to figure is 6 will work or I need to go to 8"

    What is HVAC talks thoughts?


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  2. #2
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  3. #3
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    Put in the 8" if you can - 6" will run noisier.


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  4. #4
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    how are you making up the air? it doesnt matter what size pipe you put in if there is no make up air some place.

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  6. #5
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    You should run the 8". With the 6" your air velocity will be much too high at 3200 FPM.
    "Quality is never an accident; it is always the result of intelligent effort. The bitterness of poor quality lingers long after the sweetness of low price is forgotten" --- John Ruskin

  7. #6
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    Thread Starter
    Quote Originally Posted by HVAC_Marc View Post
    how are you making up the air? it doesnt matter what size pipe you put in if there is no make up air some place.
    from the 7000 sqft of living space they have.......

  8. #7
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    Won't the cut sheet give that info?

  9. #8
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    Quote Originally Posted by mike3 View Post
    Won't the cut sheet give that info?
    they have not picked a specific model out yet. And I looked online for roughly 15 min last night and could not find pipe sizes listed in the documents online for various makes. Found one unit that was a 900 cfm unit that called for an 8" pipe but nothing much for 600 cfm units

  10. #9
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    The problems that can occur with no makeup air are 2 fold:

    1. The A/C or heat will be pulled out of the house
    2. The negative pressure in the house will be increased (which means that unconditioned outside air will be pulled in through cracks, non-tight windows, doors, etc....can also pull in air from attics, walls and other unconditioned spaces).

    @600cfm, a makeup air at the hood would be best, but a fresh air supply to the R/A of the A/C will work too. Both will be preferable, since a proper makeup air at the hood should make up less than the hood takes out (to keep smoke exhausting from space to hood). For the F.A. supply to R/A, we always use a motorized damper set to close when system is off and a FanTech filter box, placed before the damper.

    That may be more trouble than you're looking for, and it may not make much difference in your climate, but in the warm humid south, warm, humid air infiltration is a comfort killer and an air-quality no-no.

    Keep in mind that most homes are already under negative pressure if they are tight (or even if not) due to bathroom exhaust, chimney effect, etc.

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  12. #10
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    Quote Originally Posted by jbhenergy View Post
    from the 7000 sqft of living space they have.......

    buzz.... wrong answer

  13. #11
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    Quote Originally Posted by HVAC_Marc View Post
    buzz.... wrong answer
    lol I was waiting for you to come back....

    They have 3 furnaces each with high and low 10" combustion air vents. While this is not the best preferred or correct method, I would imagine they would be able to provide some of the make up air.

    As for the rest, I think they bought more then they could chew so they are already trying to cut all my cost, so they will have to make do with what is in place.


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  14. #12
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    If a range hood exceeds 450 CFM you MUST provide powered MUA slaved to the exhaust fan. Code doesn't prescribe how to do this. You also need to test for depressurization and backdrafting/ draft interference of any atmospherically vented appliances. Install unlisted low level CO monitors.

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  16. #13
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    Remember a residential range hood can also be a rectangle takeoff, usually 3 1/2 x 10 . Why I hate installing customers stuff, you don't know till you get there.

  17. #14
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    Quote Originally Posted by jbhenergy View Post
    lol I was waiting for you to come back....

    They have 3 furnaces each with high and low 10" combustion air vents. While this is not the best preferred or correct method, I would imagine they would be able to provide some of the make up air.

    As for the rest, I think they bought more then they could chew so they are already trying to cut all my cost, so they will have to make do with what is in place.


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    You need a specific designed make up air system for that amount of CFM.

    Why would you do something if you know it is wrong? That's the definition of "hackery".

  18. #15
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    Not gonna disagree there at all. But at the end of the day the person writing the check is the final decision maker.

    On a side note just to paint a picture of these people, the duct is going to be exposed until they can come up with enough money to hire a carpenter to build a soffit around it. So money is the issue....

    I can't win em all Marc


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  19. #16
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    Quote Originally Posted by jbhenergy View Post
    But at the end of the day the person writing the check is the final decision maker.
    ...
    I can't win em all Marc

    So, you're telling all of us here that you are a willing hack for the right amount of money.

    Dont you ever just walk away? I can. I "win" every job I do and I can rest easy knowing no inspector could ever complain about a job I do, even where we have NO inspections nor even licensing jurisdictions.

  20. #17
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    Marc is right


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  22. #18
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    Knowingly violating the code is severely frowned on by the courts. Should someone get injured and the proximate cause is proven to be you lack of adequate MUA, you could have criminal charges leveled against you. No, the homeowner does NOT make this call. You allow them to make an informed decision. If they choose to ignore the code you had better walk away and leave them a certified letter return receipt.

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  24. #19
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    in my area of the state most areas are not regulated. there are no licensing divisions, nor inspection services. it is my personal belief that since some of the state (in major cities near me) follows the 2008 codes, and the rest follows 2011, I should at least make an effort to AT MINIMUM follow the 2008 codes. Else, im a hack. If I dont know, I ask the town hall, if they dont know, I guess based on the code book. Anything I know to be wrong, and the customer demands it, I try to inform, and if I have to, walk away.

    most of you guys have it better. you have a statewide code and license.

  25. #20
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    In the absence of a state statute code or local ordinances the trier of fact (called "The Judge" ) will look to the most relevant national standard or code and apply it even if it wasn't voted into law locally. Hence The National Fuel Gas Code is just that. It is referenced in virtually every gas appliance mfrs. listed instructions.

    So, while I'm still under the 2009 IRC in most of PA, about 10% of the some 1270 or so jurisdictions have "opted out" of the ICC codes choosing instead legally to keep some other model bldg. code (usually BOCA around here). So, I have to cross-reference both NFPA 54 NFGC and Ch. 24 of the IRC (residential section of IFGC) plus local ordinances AND the mfrs. listed instructions. Next month a few sections of the 2015 IRC will take effect so I'll have to comply with those where they apply.

    I provide litigation support for losses due to combustion appliances so please consider our warnings. Peace.

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