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Thread: Flex Duct?

  1. #1
    Join Date
    Apr 2004
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    5
    I had an HVAC company give me some advice yesterday for adding some ductwork in my soon to be finished basement. He said it would be best to us "flex". Now I was thinking this meant the very flexible stuff, similar but more sturdy than what you connect your dryer to the exhaust outlet. But when I was in the hardware store there was some of the traditional metal duct that was labeled "flex". So which one did he mean? I know it would be easier to use the truly flexible pipe because there are so many obstacles coming from the ceiling, but is it less efficient? I am only running maybe 2 lines in to the main room.

  2. #2
    Join Date
    Nov 2000
    Location
    Waco, Texas, USA
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    6,153
    Flex duct does have more pressure drop than smooth metal pipe but that is not all to consider. It also matters how tight the flex is stretched, how many metal elbows are used, how it is wyed off. The duct board triangle wyes have a tremendous pressure drop, metal wyes are much better. Metal elbows on the boot boxes are much better too. Some supply grilles are more restrictive than others.

    You see it all depends on if the installer has this knowledge and is willing to apply it to your situation.
    "And remember my sentimental friend......that a heart is not judged by how much you love, but by how much you are loved by others" - Wizard of Oz.

  3. #3
    Join Date
    Apr 2004
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    5
    Well the installer is going to be me, they told me that it would be a waste of my money to have them install it, since I can really only tap 2 runs off of the main supply box.

    So you are saying that if I used flex, I should use a metal boot. How tight do you want the flex to be? One of the runs will just have very slight bends if I use flex, the other one will have hopefully 2 bends but maybe 3. I know you guys cant see it, but what would you suggest, the metal or the flex?

  4. #4
    Join Date
    Sep 2001
    Location
    Vancouver Canada
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    996
    Metal is always preferable to flex, if you are going to use flex duct, make sure it is only on very short runs and as straight as possible. Don't allow the flex to sag down or bend it too sharply. If possible just use metal ducting.

  5. #5
    Join Date
    Apr 2003
    Location
    Eugene, Oregon
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    1,209
    If your ducting is in an unconditioned space it needs to be insulated also. Flex ducting has a vapor bariier and insulation (r-8) attached. Metal is definatly better as erlier post suggest however it should be insulated if its underfloor or in an attic.

  6. #6
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    There is no difference in performance between ,properly sized metal and flex.There may/will be a difference in the actual sizes for the same cfm(air flow).


    chagelsk,

    when taking runs off the plenum,be sure to install an adjustable damper in each run to regulate the cfm,or they will likely overfeed,and get more cfm(air flow ) than intended.

  7. #7
    Join Date
    Apr 2004
    Posts
    5
    So, say I used the flex, my other runs for the 1st floor are 6 inch pipe, what size flex would I use. I am a pretty handy person, but have never worked with ducts before. I understand that hardest part of adding a run is cutting the hole in the plenum. But for a beginner would you suggest the flex over metal if I make sure to stretch it all the way out. Or is the metal pretty easy to work with? 1 run will be about 20 feet long and the other will be about 30. Should I put the registers close to the outside wall or in the middle of the room? To take the run to the outside will add anoth 7 or 8 feet to each run. The thing is that I am gonna have to put in some electric baseboard heat since I only have room to add 2 more runs. This basement is about 90% underground.

  8. #8
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Sizing needs info you can't provide over the net.

    I would ask the contractor that said "do it yourself".

    If it's larger than needed,who can always reduce the air flow.

    Only room to have two takeoffs:
    What about a, say,12X8 trunkline instead?You good probably take 4 6" rounds off from that size trunkline.

  9. #9
    check your building code it don't make a lick of sense but i think 14' of flex duct is all they "inspector's" will normally allow.over the past 15 year's i've ran flex duct up to 20', with no complaints.installed properly it will last as long or longer than metal,depending on factor's such as moisture etc.

  10. #10
    Join Date
    Feb 2003
    Location
    Huntsville,AL
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    4,125
    do you really think that flex will last 80+ years? I have seen metal ducts that old, maybe some 100+

    you are right, hardest part may be to cut holes for take- off, unless from round pipe. Round pipe can be cut & a wye fitting installed.

    probably next Jan- Mar, a contractor's interest in this small job will be greater.

    to get better answers about sizing, run the load calc from this site -- you will get overall equip size needed & air flow needed for each room. then a contractor & you will have facts

    if you just add ducts | outlets, you may starve other rooms

  11. #11
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Originally posted by jeffkillen
    check your building code it don't make a lick of sense but i think 14' of flex duct is all they "inspector's" will normally allow.over the past 15 year's i've ran flex duct up to 20', with no complaints.installed properly it will last as long or longer than metal,depending on factor's such as moisture etc.
    The code(s) that limit the lenght of flex,refer to flex"connectors" ,not Class 1 flex "duct".Some inspectors make this mistake.Now,I've heard some "local" codes may limit flex duct lengths.

  12. #12
    Join Date
    Jun 2005
    Posts
    64
    flex is great if used correctly. a smooth S run in flex is better airflow than 2 hard sharp turns . where its straight use pipe and never use more than 10 feet of flex in a run.

  13. #13

    another idea

    We use a product called FlexFlow Elbow quite a bit on flex duct.

    It forms flex into an elbow keeping it from kinking.

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