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Venting a range hood thru the roof

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STEVEusaPA
STEVEusaPA Member Posts: 6,505
Outside the usual HVAC, but I figure some of our fellow Wallies could have an answer.

The range hood requires a 7" pipe, and the application will be thru the roof. From the ceiling to thru the roof is uninsulated.

The instructions are a basic picture, with not much material call out. What would you guys recommend as far as:
1. Type/thickness of pipe? I'd imagine no flex.
2. Do I insulate the pipe in the attic space? I plan on sealing all joints.
3. Where would I find a roof termination kit? HD sells a 6", but it looks cheap and the instructions did say not to reduce if going thru the roof.
5. Also, how far do I have to be from a gable wall. This penetration is on the lower roof of a split level, with the upper roof following the pitch about 4' high (on a 4" pitch roof).
6. Do I (should I) provide any clean out? It's about a 5' vertical run so I'd imagine I could access it from the roof?

And thoughts, pointers, expert advice would be appreciated.
Thanks
Steve

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Comments

  • SlamDunk
    SlamDunk Member Posts: 1,596
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    I had one installed. Installers used seamless flu pipe, it is a higher guage than what you can get from big box.

    aluminum tape at all joints and elbow sections. No insulation and no cleanout. Hood has grease filters to wash periodically. Termination kit is square, plastic and four inches tall. It is cheap but functional. Hope this helps.
    STEVEusaPA
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
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    Thanks @SlamDunk. That's how I'll do it.

    There was an error rendering this rich post.

  • SlamDunk
    SlamDunk Member Posts: 1,596
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    I want to say 18-16 ga ducts. In case you have a grease fire in the stack, it wont melt.
  • rick in Alaska
    rick in Alaska Member Posts: 1,458
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    If you don't insulate it, then the heat from the pipe in the wintertime will cause a lot of condensation to form under the roof sheathing, and rot it out. My neightbor, who is a carpenter, did it that way and the OSB was black about a year later, and dripping water onto the ceiling.
    Rick
    STEVEusaPA
  • ChrisJ
    ChrisJ Member Posts: 15,870
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    SlamDunk said:

    I want to say 18-16 ga ducts. In case you have a grease fire in the stack, it wont melt.

    16 to 18 gauge!

    Why are you going so cheap, might as well use 10 gauge at that point.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • mattmia2
    mattmia2 Member Posts: 9,850
    edited November 2022
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    Look on broan/nutone's web site/catalog, you will get the part number for the roof termination in the size you need then you can find a supplier for it. I think supplyhouse.com carries that stuff. There are also extra tall versions to get over the snow cover on the roof.

    I think the residential code will have the spec for the pipe.

    I reduced mine to use an existing termination I had but I went from like 10" to 7" or 8" because I wasn't super concerned about getting the full airflow at high speed, i just wanted something that could pull the smoke from smoky cooking operations outside.
  • mattmia2
    mattmia2 Member Posts: 9,850
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    It looks like the cap and the flange that goes through the roof are separate parts now, when I installed one 20 years ago it all came as a kit, you installed the flange through the roof and the cap sat on top of the flange.
  • DJD775
    DJD775 Member Posts: 252
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    If you don't insulate it, then the heat from the pipe in the wintertime will cause a lot of condensation to form under the roof sheathing, and rot it out. My neightbor, who is a carpenter, did it that way and the OSB was black about a year later, and dripping water onto the ceiling.
    Rick

    Are you sure this was an issue with insulation and not a duct sealing issue? In a residential install I can't see the duct getting too warm as you are pulling in mostly room temperature air in addition to what ever is created by the pan or two you are using for cooking.
    mattmia2
  • mattmia2
    mattmia2 Member Posts: 9,850
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    The knock it out as fast and cheap as possible types will just stick the duct through the roof and put a roof vent over it or maybe put a cap with a damper but you really should have the duct through a sheet metal plate that sits under the cap and isolates the duct from the roof.
  • ChrisJ
    ChrisJ Member Posts: 15,870
    edited November 2022
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    DJD775 said:

    If you don't insulate it, then the heat from the pipe in the wintertime will cause a lot of condensation to form under the roof sheathing, and rot it out. My neightbor, who is a carpenter, did it that way and the OSB was black about a year later, and dripping water onto the ceiling.
    Rick

    Are you sure this was an issue with insulation and not a duct sealing issue? In a residential install I can't see the duct getting too warm as you are pulling in mostly room temperature air in addition to what ever is created by the pan or two you are using for cooking.

    I don't understand how a warm pipe causes condensation on the outside in the winter.
    Condensation on the inside, running back down the pipe absolutely, but outside of the pipe?

    This is why I insulate bathroom fan ducting, to reduce condensation inside of it in the winter. I suppose it could sweat a lot in the summer if the dew point is really high.

    But I'm not understanding the winter outside pipe sweating concern.

    That being said....
    I believe weird stuff happens in places like Alaska, like needing oversized sewer vents due to frosting etc so maybe he means the outside of the pipe gets cold enough to sweat in the attic?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
    DJD775