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  1. #14
    Join Date
    Mar 2008
    Location
    Murfreesboro TN
    Posts
    9

    long flex runs

    I've found long flex runs can create substantial loss. I have worked for a couple companies and learned a few tricks about flex. One particular job, we had flex a couple feet out of the trunk, hard pipe about 10', and then flex to the boot. Flex, if not installed right and pulled tight, will create loss and higher statics. High statics, if extreme, can cause problems in the unit.

    Go with the short flex runs at the boot as this will eliminate some noise. Use as much metal as you can.

  2. #15
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by n4hen View Post
    I've found long flex runs can create substantial loss. I have worked for a couple companies and learned a few tricks about flex. One particular job, we had flex a couple feet out of the trunk, hard pipe about 10', and then flex to the boot. Flex, if not installed right and pulled tight, will create loss and higher statics. High statics, if extreme, can cause problems in the unit.

    Go with the short flex runs at the boot as this will eliminate some noise. Use as much metal as you can.
    Flex sized by using Manual D,will be no more restrictive to air flow than metal sized by Manual D.

  3. #16
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,296

    Red face

    Quote Originally Posted by Jake1980 View Post
    I am using sheet metal for the new runs into each room.

    Should I use flex duct in the last 1' - 1.5' that attaches to the registers?
    Thanks, Jake
    Sounds Kinky to me
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  4. #17
    Join Date
    Jan 2008
    Location
    MI
    Posts
    38
    It’s really kinky. Hay jake1980 you can use up to six foot of flax, than it will be really easier. But why are you questioning what you have been told by your company on a first place?

  5. #18
    Join Date
    Nov 2003
    Location
    northern mass
    Posts
    411
    common for myself......hard pipe the whole way untill i have to cut a 5'. then I flex to boot. All my flex is (obviously) under five feet. I really like it this way.
    In attic....paremeter duct, then flex to runs....probably an average of maybe three feet ?? simple.

  6. #19
    Join Date
    Feb 2008
    Location
    Missouri
    Posts
    55
    Quote Originally Posted by gena View Post
    It’s really kinky. Hay jake1980 you can use up to six foot of flax, than it will be really easier. But why are you questioning what you have been told by your company on a first place?
    my company is saying 15'-20' of flex is okay...
    Although all my runs are probably @ 10' with the exception of 1 run.

    I was going to go with hard duct and then when looking at how the runs would have to run it would fill up my attic and make it hard for anyone to ever work up there, i.e. cable company, electrician. Alot of ductwork would be in the way of the path they would have to go.

  7. #20
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,167
    Sized properly, flex is ok for runs longer then 20'.
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    How many times must one fix something before it is fixed?

  8. #21
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    Quote Originally Posted by scottsacavsfan View Post
    Judy,
    If it where my house and I was goingto be their a while I would spare no expense go hard pipe and insulate the entire lenght
    if you think you may be out in a few years the flex it

    As far as easy to paint my celing registers are very easy to remove 2 screws and they fall right out
    problem is if the flex is not anchord down correctly then when it comes time to reinstall the registers the flex tends to flop around in the attic and make it very hard to get the register back in the boot
    I have had to climb up in the attic and place board on the boot to hold in plce so I could go back and put the register in
    If sealing boots to the ceiling drywall were commonplace, the boot would stay put when you're installing the register...especially with mastic used as the sealant. I've sealed boots to the ceiling with mastic...once it dries that puppy ain't moving!

    Sealed boots also have the benefit of reducing conditioned air loss to attic spaces above, and reducing interior stack effect.

  9. #22
    Join Date
    Dec 2003
    Posts
    709
    In an attic, I would use flex all the way. I have been in attics that have the first flex ever made I believe and it was still intact. Just a wire helix insulation and a grey plastic jacket. At least 30 years old. I always get my hard round trunkline as close as possible to the runs.

  10. #23
    Join Date
    Mar 2008
    Posts
    657

    boca code

    i could be wrong but i believe boca code states that flex runs longer then 12 ft. are not acceptable.

  11. #24
    Join Date
    Dec 2003
    Posts
    709
    My local code says no longer than 14'. I design the duct system to #1 work properly and efficiently as possible, and #2 keep material and labor cost down. A few feet of flex beyond what the code states doesnt keep me up at night.

  12. #25
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,167
    Are you reading the code for flexable duct connectors.
    Or for flex duct.
    A flexable duct connector, is NOT flex duct. They are 2 different things.

    International Code says 14' for flexable duct connector.
    It has no limitation on flex duct.
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    How many times must one fix something before it is fixed?

  13. #26
    Join Date
    Jan 2008
    Location
    MI
    Posts
    38
    It's funny, but this is a Flexible Duct Connector! Do your homework!
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