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Thread: Duct sizing

  1. #1
    Join Date
    Jun 2017
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    Duct sizing

    Hey everyone, I am installing a 111,000 btu furnace/central air in a fully insulated and finished pole barn with concrete floors. There is no existing ductwork, so I'm planning on placing the furnace in the center of the building and installing spiral duct off the plenum in both directions, along the ceiling, for the length of the building. It is one giant open room, about 2800 sq feet.

    My calculations have come up with 18" spiral duct to handle 1500 cfm. My question is, since I am splitting right off the plenum in opposite directions, can I size the ductwork smaller/do I split the cfm required by each duct then?


    It may be a commercial building (winery) someday, but for now is just a pole barn on a residential property. The space will remain one giant room in any case.

  2. #2
    Join Date
    Aug 2002
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    Southold, NY
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    Quote Originally Posted by avetsch View Post
    Hey everyone, I am installing a 111,000 btu furnace/central air in a fully insulated and finished pole barn with concrete floors. There is no existing ductwork, so I'm planning on placing the furnace in the center of the building and installing spiral duct off the plenum in both directions, along the ceiling, for the length of the building. It is one giant open room, about 2800 sq feet.

    My calculations have come up with 18" spiral duct to handle 1500 cfm. My question is, since I am splitting right off the plenum in opposite directions, can I size the ductwork smaller/do I split the cfm required by each duct then?


    It may be a commercial building (winery) someday, but for now is just a pole barn on a residential property. The space will remain one giant room in any case.
    Manual "D" for duct design.

    How did you come up with 111,000 BTU's?

  3. #3
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    Jun 2017
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    Thread Starter
    I'll check it out thanks! It is 110,000 btu furnace, I fat fingered it.

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