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I am installing an under the cabinet range hood. I can't vent directly out the back because there is a stud in the way. So I'm going up into the cabinet, making a turn, and then out through the exterior wall.

My range hood manufacturer calls for 7" round pipe. (or 3.25"x10x rectangle but that is not possible)

Cabinet space is tight, the metal ducts might not fit. So it made me wonder two things, which I cannot find answers for:

  1. Is it ok to reduce to 6" instead? How bad could that be?
  2. Is it ok to use pvc pipe? Seems like heat would never be an issue with a range hood exhaust. If so... does anyone know where to find 7" pvc pipe? Not at home depot or lowes, and I'm not having luck searching the internet either.

Thank you

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  • Some AHJ allow heat rated pvc. Reducing the diameter will affect the cfm rating and increase the strain on the blower motor also makes more noise. what is the cfm rating on your unit?
    – Kris
    Jan 13 '20 at 19:37
  • can't you replace the stud with a header build-up, like adding a small window? There's an AskTOH where tommy gets stuck at the site until dark after finding one in the duct path behind the homeowner's microwave, and he shows how to handle such surprises.
    – dandavis
    Jan 13 '20 at 19:56
  • 2
    Heat isn't the issue, it's that PVC is absolutely awful if a grease fire reaches into (or starts in) the vent. Jan 13 '20 at 20:44
  • 6" is only 73% the area of 7" (or 27 percent smaller, if you prefer) - and yeah, when the grease lights up, you do NOT want it to be in PVC. Also, PVC will typically be LARGER than the same size metal (thicker wall.) Headering the stud out of your way is an excellent idea and might want to be an answer, not a comment, @dandavis
    – Ecnerwal
    Jan 13 '20 at 21:28
  • Makes sense - thank you Matthew et al. I will stick with 7" metal (hood is 265 CFM). Don't want to move the stud because it would entail cutting through the backsplash unfortunately. I worked hard on that backsplash!
    – ojef1
    Jan 14 '20 at 16:38
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Check your local building codes to be 100% sure but PVC is NOT permitted by many. I'll reference one from TX but most are similar where it says:

M1503.2 Duct Material

Ducts serving range hoods shall be constructed of galvanized steel, stainless steel or copper.

Regarding size, I believe you'll find that your local codes also require adhering to the hood manufacturer's specification for duct size.

See: TX Code

2
  • The ICC code says the same...
    – Lee Sam
    Jan 13 '20 at 19:44
  • @LeeSam I figured that it might since most local codes are based on that.
    – jwh20
    Jan 13 '20 at 19:46

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