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Thread: kitchen exhaust vent question

  1. #1
    DIY Junior Member astro46's Avatar
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    Default kitchen exhaust vent question

    I know, this really isn't an hvac question, the only relationship may be the use of sheet metal. Nevertheless: I want to install an undercabinet microwave vent hood combo over kitchen range. the current range hood (no microwave) exhausts into cabinet above, 6" rd vent, 2 90's, into wall, about 10" right of center, and up to roof.

    the new microwave will require raising the cabinet about 6". I don't think that the current 6" rd 90's will fit any longer. Possibly, using 4" 90's may work. The other option is to have a custom plenum made so that exhaust hood goes up into plenum and plenum out into wall (6" duct to roof). two questions:
    1. would it be ok to use 4" out of the exhaust then into the existing 6" to roof?
    2. anything wrong with the plenum idea? better, worse, no difference, than using 4" duct (assuming 4" duct will accommodate the curves)?

    Thanks,

    Jeffrey

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Each hood device has a section in the installation instructions that says what diameter duct it needs and the maximum 'effective' length. The effective length is different than the actual length because every time you change direction, it creates more restrictions, making the run appear longer. Without knowing the model and what size it is designed for, there's no way to know if what you want to do will provide proper ventilation. In general, smooth curves and large diameter is what is the most efficient.
    Jim DeBruycker
    Important note - I'm not a pro
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    DIY Junior Member astro46's Avatar
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    Right. I am aware of these instructions. Keeping the venting at 6" would be ideal. Short of opening the wall, plus dealing with possible obstructions that precluded putting the vent in the center in original installation, I am working with what I have.

    Question remains: is substituting 2 4" 90's for the existing 2 6" 90's going to be the "end of the world", or would a plenum be so much better that it is worth the extra time and money it would take to implement it. And, is there some reason why a plenum is not advisable in a kitchen exhaust duct. I know that they are used in hvac systems.

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    Jack of all trades DonL's Avatar
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    Don't forget to install the fire extinguisher.


    Good Luck.
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    DIY Senior Member guy48065's Avatar
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    Quote Originally Posted by DonL View Post
    Don't forget to install the fire extinguisher.


    Good Luck.
    Increasing your post count?
    Romeo and Atlanta, MI

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Again, without knowing the max effective length of the duct THAT system can support, there's no way to know if the 4" section with those elbows would take it over the maximum length. In general, restricting the diameter and changing directions a few times is a very bad idea. Keep in mind that the filters aren't perfect, and those extra elbows just become great grease traps. A plenum, properly designed, is better if you can avoid the 4" and elbows.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member astro46's Avatar
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    Jim, the actual length of the run is ap. 7 ft. currently has 3 6" 90 deg elbows in the run. From what I remember of microwave installation charts this is well below the recommended max. I suspect that substituting 2 4" 90's for 2 of the 3 current 6" 90's would not have an extremely negative effect on the operation Though, if there is a good chance that reducing the diameter of 90's coming out of the exhaust converter would cause a grease trap , and having a plenum is acceptable, I may go that route. Or open the wall and cut 6" off the existing vertical and use the system as currently constructed; more work, but back in operation faster than waiting for a plenum to be made. (Simplest of all, though, would be using the smaller radius of 2, 4" 90's).

    btw: does going from 4" to 6" have a negative effect on air flow, or increase grease collection? I would think that the air flow would be happy getting to 6" duct on its way out.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The larger the pipe, the quieter it will be and the more cf/m airflow you'll get (which is good). Any place there is turbulence or a reverse slope is a potential collection point. Going from 4" to 6" won't hurt, but again, it all depends on the max effective length the system was designed for. Some are surprisingly short, some aren't, and elbows add up very quickly.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by guy48065 View Post
    Increasing your post count?

    Not really, Just thinking Safety First. Is that wrong ?

    Depending on the way it is done grease can be a problem.

    The Microwave will also need to have the proper ventilation, without grease building up in it.


    Have Fun.
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  10. #10
    DIY Junior Member astro46's Avatar
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    Quote Originally Posted by jadnashua View Post
    The larger the pipe, the quieter it will be and the more cf/m airflow you'll get (which is good). Any place there is turbulence or a reverse slope is a potential collection point. Going from 4" to 6" won't hurt, but again, it all depends on the max effective length the system was designed for. Some are surprisingly short, some aren't, and elbows add up very quickly.
    quiet and perhaps 4" causing noticeable decrease in air flow would be good reasons to see about opening the wall and shortening the vertical. so, if all goes well that will be the plan. otherwise I will look at using 4" elbows between the microwave and 6" vertical.

    thanks for the info.

    Jeffrey

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