kitchen exhaust vent question

Discussion in 'HVAC Heating & Cooling' started by astro46, Sep 29, 2013.

  1. astro46

    astro46 New Member

    Messages:
    24
    Location:
    Chicago
    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
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
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    Location:
    New England
    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.
  3. astro46

    astro46 New Member

    Messages:
    24
    Location:
    Chicago
    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.
  4. DonL

    DonL Jack of all trades

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    Location:
    Houston, TX
    Don't forget to install the fire extinguisher.


    Good Luck.
  5. guy48065

    guy48065 New Member

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    Location:
    SE and north MI
    Increasing your post count?
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
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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.
  7. astro46

    astro46 New Member

    Messages:
    24
    Location:
    Chicago
    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.
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    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.
  9. DonL

    DonL Jack of all trades

    Messages:
    3,815
    Location:
    Houston, TX

    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.
  10. astro46

    astro46 New Member

    Messages:
    24
    Location:
    Chicago
    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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