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  1. #1
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    Nov 2009
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    Simple CFM/Flex duct qustion?

    How much CFM would a 16 inch flex duct carry over 35-40 foot maybe a little more. My ductaltor is showing at 25 foot to be able to carry 1000 cfm. Once the 25 foot is passed or extended how much percentage is fair to take off so would have what that duct will carry with this length.

  2. #2
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    Jun 2010
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    Philadelphia PA
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    Ductulator is for sheet metal ONLY

    unless you happen to have one from a flex manufacturer. Haas to be the same Company and PRODUCT that you are using or the info not accurate.

    In general, flex has 2x the resistance. think its a big deal?
    In general agian, 16" would need to be 18"

    Check ACCA Man D for more info
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  3. #3
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    Jan 2010
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    Athens GA
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    1,234
    genduct,your info is correct but only if its installed correctly.The only problem with that is only one out of every five hundred flex jobs is installed correctly.
    On a 25-40 ft length I would only use a metal duct.There just isn't much way of getting flex stright,and tight.

  4. #4
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    Jul 2006
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    Virginia
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    4,887
    probably about 750 depending on how kinked it is installed

  5. #5
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    Nov 2009
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    West Monroe, LA
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    Quote Originally Posted by catmanacman View Post
    probably about 750 depending on how kinked it is installed
    That is what I was thinking due to the length of run on the 16 inch. I am looking at my bosses system. As it stands right now he has (1) 20x25 return in the ceiling that is right unit which is in the attic. So he getting somewhere in the range of 1000cfm at the return. He current system is a (5) ton. After doing load on the home, I realized the correct size system is a (4) ton so the company will be installing the new (4) ton in the next week.

    My plan is having the techs add another 20x25 return at the other end of the home & running the 16 inch back to the return air box in the ceiling to give this system proper return and to increase comfort on that side of his home as well. It amazes me that a contractor would have installed only (1) 20x25 return on a (5) ton system, shoot even on a (4) ton that is crazy.

  6. #6
    Join Date
    May 2008
    Location
    Central Fla.
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    311
    Every ductilator I have seen is based on 100ft. of friction. with that said it should move 1,400 cfm.

  7. #7
    Join Date
    Jul 2004
    Location
    Massachusetts
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    6,841
    Why would you even consider running flex duct over that length? That's just not right. In reality, flex duct doesn't deliver the same CFM's of rigid duct because in order to do so or even come close, the flex must be stretched tight. That means straight, pulled tight to anchored fittings at each end. I've never seen flex duct properly installed when the minimum diameter is used. For those who use flex for shorter runs and/or connections to boots, most will use a larger diameter than what is minimum to allow for pressure loss from the slack duct. Put in rigid and use short lenths of flex to make the connections is needed. AT 16-inches, I fail to see the need though.
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  8. #8
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    Jan 2004
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    PA
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    Quote Originally Posted by adamwhatley View Post
    How much CFM would a 16 inch flex duct carry over 35-40 foot maybe a little more. My ductaltor is showing at 25 foot to be able to carry 1000 cfm. Once the 25 foot is passed or extended how much percentage is fair to take off so would have what that duct will carry with this length.
    A 16" flex will easily carry 10,000 CFM at about a 17" FR(yep, it'll blow apart after a very short time ).

    So may be if you asked how much air at the FR you need, the question could be answered a bit more accurately.
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  9. #9
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    Jul 2006
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    Virginia
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    Quote Originally Posted by TwincamDave View Post
    Every ductilator I have seen is based on 100ft. of friction. with that said it should move 1,400 cfm.
    A balometer will show you different

  10. #10
    Join Date
    Mar 2011
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    90
    Quote Originally Posted by skippedover View Post
    Why would you even consider running flex duct over that length? That's just not right. In reality, flex duct doesn't deliver the same CFM's of rigid duct because in order to do so or even come close, the flex must be stretched tight. That means straight, pulled tight to anchored fittings at each end. I've never seen flex duct properly installed when the minimum diameter is used. For those who use flex for shorter runs and/or connections to boots, most will use a larger diameter than what is minimum to allow for pressure loss from the slack duct. Put in rigid and use short lenths of flex to make the connections is needed. AT 16-inches, I fail to see the need though.
    I have to agree with you that rigid are better than flex, but installing rigid ducts are a b1tch, especially for return duct. Since he already has a 20 X 25 X 1 return. adding a 16' flex should be more than enough for his 5 ton even at 40 foot, just make sure there is no kinks/restrictions.

  11. #11
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    Nov 2009
    Location
    West Monroe, LA
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    I will agree metal would be the best but not cost effective. The new system will be a 4 ton. So the system will be sized correct. So even with a 4 ton (1) 20x25 is not enough return air, I think we all agree on that. That is the reason for adding another 20x25 return & the only location we can install is going to around 40 foot. The new return will allow a straight shoot to the return in the attic. A 14 inch with a short return sould be a enough but since the distance is so far that why going to upsize to 16 inch.

  12. #12
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    A 14" flex is more appropriate for a 1.5 ton system, so it would not have been really enough for a second return for your 4 ton unit.
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  13. #13
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    Nov 2009
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    West Monroe, LA
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    Quote Originally Posted by beenthere View Post
    A 14" flex is more appropriate for a 1.5 ton system, so it would not have been really enough for a second return for your 4 ton unit.
    Funny you said that beenthere. His upstairs unit is a (1.5) ton unit and the return for it runs about 10 foot and at current it is a 12 inch flex. We are going with XL 20i systems so had to go to a 2 ton for the upstairs, techs will be increasing to 14 inch as the length is not very long and to honset about all we can fit in the little area.


    Know as far as the (4) ton system goes I will have them run a 16 inch as the above stated comment and due to the distance. I feel the 20x25 current return along with the new 20x25 return at the other end ran with 16 inch flex should be more then enough return for the system.

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