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  1. #1
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    Flex ducts and CFMs

    do these sound right for cfm rating of flex duct?

    for instance I have been told 6'' flex is 110 cfm

    this says 6'' flex is 75 cfm
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  2. #2
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    the cfm depends on the static the system is based upon. Usually a 6" round is about 120cfm using galvanized pipe. With flex pipe, there is a greater resistance to airflow so the cfm may be less with a higher pressure. I would anticipate that you would get somewhere between 100 and 120 cfm through the 6" flex. It's not that 6" flex or round pipe "is" or "isn't" a certain cfm. The size pipe is 'capable' of a certain airflow at a certain static pressure.

    Can you expand on the context of your question?

    All the best, John.
    Everyone has a purpose in life..........even if it's to be a bad example.

    Seek first to understand, before seeking to be understood.

  3. #3
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    There is also a FPM limit for each type of pipe. I'm not sure why (noise, turbulence, duct abrasion...), but Manual D lists these limits. Smooth metal duct has the highest limit, and flex the lowest.

    So there are two limits: velocity (feet per minute), and static pressure loss. You can't exceed either one.

  4. #4
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    Cfms for 6" duct are what they are .depending on the duct system.


    First a Manul D to determine the actual Friction Rate (FR)when designing the duct system.

    6" round metal
    @.04FR=67 cfms

    @.06FR=82cfms

    @.08FR=96cfms

    @.10FR=107cfms


    If flex it wil be less cfms in each case.


    The more resistance,turns,length,etc., that the duct system has the Lower the FR will be.

  5. #5
    Join Date
    Jan 2006
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    There is a ductulator for flex.

  6. #6
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    Quote Originally Posted by onearmsteve View Post
    do these sound right for cfm rating of flex duct?

    for instance I have been told 6'' flex is 110 cfm

    this says 6'' flex is 75 cfm
    doesn't sound too far off to me...I deduct 25% airflow when going from solid to flex and run 7" runs instead of 6"

  7. #7
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    Quote Originally Posted by dash View Post
    Cfms for 6" duct are what they are .depending on the duct system.


    First a Manul D to determine the actual Friction Rate (FR)when designing the duct system.




    If flex it wil be less cfms in each case.


    The more resistance,turns,length,etc., that the duct system has the Lower the FR will be.
    To the right I added in the Flex CFM @ the same Static

    6" METAL /////// FLEX
    @.04FR=67 cfms 52 cfm

    @.06FR=82cfms 64 cfm

    @.08FR=96cfms 74 cfm

    @.10FR=107cfms 82 cfm

  8. #8
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    Quote Originally Posted by coolguysfl View Post
    To the right I added in the Flex CFM @ the same Static

    6" METAL /////// FLEX
    @.04FR=67 cfms 52 cfm

    @.06FR=82cfms 64 cfm

    @.08FR=96cfms 74 cfm

    @.10FR=107cfms 82 cfm
    Nice specs....how many feet are those based on?.....like a ductulator is based on per 100 feet after calculating in turns and elbows to arrive at a distance. The reason I ask is that I couldn't imagine running 100 feet of running flex on a run and figuring 15 feet per turn and hoping for even achieve 82 cfm
    Last edited by tinmantu; 01-26-2007 at 07:03 PM.

  9. #9
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    The FLEX #'s I posted are right off the FLEX ductulator. Flex has different rules:

    Flex Sizing is based on 20' of EQUIVILENT duct

    EQUIVILENT means each 90 = 10' , when 40' EQUIVILENT is met, you have to increase duct size X 1 size... 2' or less = decrease X 1 size

    Most of the installations I find with radically excessive duct statics are cause by misinformed contractors using a regulator DUCTALATOR instead of a FLEX ductulator.

  10. #10
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    I wish I had access to a flex ductulator so that I could prove to my boss why I want to over size runs instead of going with tin standards ....all I can try to do is use logic. Many times logic loses to profit.

  11. #11
    Join Date
    Nov 2004
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    Quote Originally Posted by tinmantu View Post
    I wish I had access to a flex ductulator so that I could prove to my boss why I want to over size runs instead of going with tin standards ....all I can try to do is use logic. Many times logic loses to profit.
    Disturbing but true ----

    The data is available @ www.atcoflex.com
    or call them @ 800-877-3828 and buy a ductulator

  12. #12
    Join Date
    May 2005
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    Western Kansas
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    Thank you for the link coolguysfl!

  13. #13
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    Quote Originally Posted by coolguysfl View Post
    The FLEX #'s I posted are right off the FLEX ductulator. Flex has different rules:

    Flex Sizing is based on 20' of EQUIVILENT duct

    EQUIVILENT means each 90 = 10' , when 40' EQUIVILENT is met, you have to increase duct size X 1 size... 2' or less = decrease X 1 size

    Most of the installations I find with radically excessive duct statics are cause by misinformed contractors using a regulator DUCTALATOR instead of a FLEX ductulator.

    Though your flex cfms posted is about the same as an ACCA flex ductolator,I cna't agree about being based on 20' of equivilent feet,it's based on 100' with theirs.

    Now atco may have a method ,it's not Manual D,may work fine but so does Manual D.

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