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Thread: Flex Duct

  1. #1
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    Mar 2005
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    In a crawl space environment should insulated flex duct be left on the ground that is protected with a vapier barrier or should the flex duct be supported from the floor joists? If the flex duct should be supported from the floor joists what is the best way to provide adequate support and what is the proper lenght between the supports.

    In an attic environment should insulated flex duct be above or below blown-in insulation.

    Thanks in advance


  2. #2
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    Apr 2003
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    Flex in crawl must be off ground and supported at least every 4 feet per man. instructions however it must be supported to keep it straight. In the attic If properly supported I prefer under/in the blow in, but some juristsions or inspectors won't allow it.

    [Edited by hvac-tech-lane on 03-12-2005 at 03:16 PM]

  3. #3
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    hvac-tech-lane

    thanks for the reponse - what is the best was to support it off the ground?

    thanks

  4. #4
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    Flex duct hanging strap.

  5. #5
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    Sep 2004
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    3,157
    Whatv would be an acceptable method of attaching fittings to Flex?
    The method I have seen has been to pull out the inner liner( after cutting to length) and taping the inner liner to the flex and taping the outer and inner to the fitting

    there has to be a better method!

  6. #6
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    BOCA AND Icc call that flex duct cannot be ran more than 12 Ft.

  7. #7
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    May 2004
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    Big, beg to differ, Flex duct has no length maximums since it is a Class 1 duct, but flex connector has the 14' rule. Big difference between the two.
    Vern P: 2003 MBC,MRC,IFGC,IFC
    An HVAC-Talk Michigan Chapter Mechanical Inspector, Jurisdiction-Ann Arbor

  8. #8
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    Jan 2005
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    Seattle
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    Hmm Flex

    Big. What is "BOCA AND Icc call that flex duct cannot be ran more than 12 Ft".

    Vernp. What is the"flex connector 14' rule"

    Flex is suppost to supported every four feet min.Per manufactures specs(some say 5').THe duct is to be supported such that the "sag" does not exceed 1/2"per foot of spacing.Inside radius of any turn shall be no less than 1/2 the diameter.Hanging strap used shall be no less 1 1/2" wide.Connections to any fitting is to be with a UL 181b-fx tape with a min of two complete wraps.
    Hope this helps.
    41GASMAN

  9. #9
    Join Date
    Mar 2005
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    28
    41gasman

    Back to the crawl space application of flex duct - using the flex duct hanging straps - do you secure these straps to the floor joists? and if so what is the best application for this process.

    I am having my HVAC system replaced and the existing flex duct was never supported - unfortunately it is laying on the ground - I want to make sure the next contractor does this correctly - he is going to replace all flex duct but never said anything about properly supporting the flex duct

    thanks

  10. #10
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    Apr 2003
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    When hanging flex I use either roofing nails through a folded over twice tab of the strap, or about 3, 1/2 inch crown by 5/8 leg 16 gauge staple shot with a senco air gun.
    For attaching duct about 4 or 5 wraps of tape with a half over lap extending the tape at least 1 full width onto the collar or fitting, and extending past the end of the metal for the liner then a duct zip tie over the insulation centered at the tape joint.

  11. #11
    Join Date
    Mar 2005
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    28
    hvac-tech-lane


    Many thanks

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