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Flexible HVAC Duct

| Posted in General Discussion on January 4, 2000 11:57am

*
What is the experience with flexible ductwork for residential use?? I’m talking about the type with a mylar liner and ‘slinky-like’ wire inner core surrounded by approx. 1″ fiberglass insulation and outer polester/mylar wrap. Talking about comparison to rigid sheet metal duct. Also looking only at use in branch ducts, the main (trunk) ducts remain as sheet metal.

Some advantages: Noise deadening (quieter) Insulated Continuous inner liner

Disadvantages: Higher resistance to air flow.

Also, doesn’t seem that there is a good technique for flexible to round sheet metal duct connections. Mfrg recommends duct tape which I wonder how long before it dries out.

Looking for real-world experience (good or bad).

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  1. Guest_ | Dec 07, 1999 10:32am | #1

    *
    Walter, in my neck of the woods, NE Florida, metal ductwork is primarily used in commercial application. All new construction and remodeling and add-on HVAC in residential applications that I have personally seen is ductboard plenum with a large diameter flexduct run to another ductboard distribution box. Individual flexduct runs are then made to each outlet from either the main plenum attached to the airhandler or from the distribution box, or both, depending on the calculations. You probably know that flexduct is a piece of cake to install and one company here advertises complete installation in one day.

    There is duct tape from the big orange box and then there is duct tape from the HVAC distributor. Big difference in stickiness and gauge. Two complete wraps required for sealing the mylar to the starter collar then a plastic tie. Outside covering with insulation pulled tight and lashed with plastic tie. Also mastic on the ductboard to collar connection.

    HVAC companies using flexduct don't have to have a bunch of sheetmetal mechanics (read $$$$$$) on the payroll, just mostly a bunch of skinny guys who can crawl thru attics and are immune to the heat. Anybody (well, almost) can be trained to cut and assemble ductboard. It's cheap enough so you don't lose your shirt when they measure once and cut twice. Off the top of my head I seem to remember the wholesale price of one box of 6 inch flexduct (25 ft), one insulated boot, one starter collar and one cover being something less than $25. Installation (mat. incl) starts at about $120 per run (retail). This doesn't include the basic system installation with plenum(s). And, of course, flexduct is exactly that, FLEXIBLE. It will go through spaces where you wouldn't dream of putting in sheetmetal.

    I have seen installations where, for some reason, the outer wrap has split the length of the duct, causing the insulation wrap to fall away from the inner liner. Still don't know if this was a manufacturer's defect or some real cheap, cheap product or an installation snafu.

    Does this shed any light or answer your question?

    Ralph

    1. Guest_ | Dec 07, 1999 06:19pm | #2

      *Walter- have ceiling cable heat with no ductwork. Nine years ago had a HVAC company install air and heat pump. They used the ductwork you are talking about and made all the runs in the attic and drops from there. The tape they used is thinner and stronger than ducttape. Very sticky. I have been in the attic recently and can tell you that the tape and duct is still like new. The system is very quiet - they called it " whisper duct ". I am very pleased with all of it. Hope this helps. Jared

      1. Guest_ | Dec 08, 1999 07:55am | #3

        *The "Duct Tape guys" have a web site at: http://www.octane.com/ducttape/index.htmlbut I am not sure what they can do about the following report from the Lawrence Livermore Lab (the Atom Bomb guys..."):"You can keep your trouser cuff out of your bicycle chain with duct tape; if you need a money belt, you can use it to strap yourmoney to your tummy. Some people claim they can cure warts with it. Unfortunately, one of the things you can't do with ducttape is seal ducts. At least not for long... "Thus the Lab alerted the media that Max Sherman and Iain Walker of the Environmental Energy Technologies Division (EETD)had thoroughly tested numerous kinds of duct sealants--and found only duct tape wanting. "We tried as many different kinds of duct sealants as we could get our hands on," says Sherman, who heads EETD's EnergyPerformance of Buildings Group. "Of all the things we tested, only duct tape failed. It failed reliably and often quitecatastrophically." http://www.lbl.gov/Publications/Currents/Archive/Sep-4-1998.html#RTFToC1I would appreciate any specific names of duct sealants, particularly tape, that is superior to the traditional stuff.

        1. Guest_ | Dec 08, 1999 04:02pm | #4

          *Walter,I have no problem with flex duct for your individual runs. We usually use sheet metal for the trunk lines. Make sure that the flex runs are not pulled tight around cornors. A routine inspection can tell if it becomes kinked. We mastic all joints. Rick Tuk

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