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Thread: B-Vent size

  1. #1
    Join Date
    Oct 2005
    Location
    MN
    Posts
    133

    B-Vent size

    I currently have an 8" B-Vent flue with about 30' of heigth. It is serving 2 nat. venting gas furnaces (160,000 and 108,000 btu) and a 75,000 btu nat venting gas water heater. If I have the 160,000 btu furnace changed out and vented with PVC will the flue have to be lined to a smaller size, or can it remain at 8" while only serving the remaining 183,000 btu max.

    Thanks

  2. #2
    Join Date
    Mar 2001
    Posts
    1,213
    Go with what your contractor says - he/they will have to deal with the AHJ (authority having jurisdiction)

  3. #3
    Join Date
    May 2001
    Location
    Kansas City
    Posts
    2,729
    As long as both the furnace & WH are natural draft hoods 8" is still ok.

  4. #4
    Join Date
    Oct 2005
    Location
    MN
    Posts
    133
    I looked at a couple B-Vent sizing charts and in both charts there was only a MAX btu for natural drafted, no minimum listed. Fan drafted has both a min. and max. SO am I to assume there is no btu minimum for natural drafted?

  5. #5
    Join Date
    Jan 2006
    Location
    Powell River, BC, Canada
    Posts
    762
    Quote Originally Posted by frankt View Post
    I currently have an 8" B-Vent flue with about 30' of heigth. It is serving 2 nat. venting gas furnaces (160,000 and 108,000 btu) and a 75,000 btu nat venting gas water heater. If I have the 160,000 btu furnace changed out and vented with PVC will the flue have to be lined to a smaller size, or can it remain at 8" while only serving the remaining 183,000 btu max.

    Thanks

    My tables show 2 spillage succeptable (nat draft) appliances with 183,000 btu input and 30' common vent height and double wall (b-vent) vent connectors needs a minimum of a 6" b-vent.

    In the general venting requirments, it states "When the table permits more than one diameter of pipe to be used for a vent or vent connector, the smallest permitted diameter shall be preferred."

    I would take that to mean you need a liner. I would assume too large of a vent would cause condensation and backdrafting issues. Your local codes and inspectors may be different though, so check with the authority having jurisdiction in your area.

    YMMV
    Where are you? Are you done yet? I got ONE more call for you.....

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