Flex duct *better* than hard duct??
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  1. #1

    Flex duct *better* than hard duct??

    I am considering bids from three contractors regarding complete replacement of my A/C and heating in Houston, TX townhouse. The inside equipment and ducting will be in the attic, which is typical for this area. My current ducts are 25-yr-old gray plastic-wrapped flex, and I'm including their replacement in my bid requests.

    One contractor insists on installing R8 insulated hard duct, with all joints throughout the system sealed with "Uni-Mastic 181".

    The other two contractors install flex duct.

    Going into this, I had in my head that "hard ducts are better, but you gotta pay big $$ for them". I think I got that from reading threads on this board.

    However, one of the flex guys tells me, "Yeah, in a theoretical system where duct size is constant and the system is perfectly sealed, hard ducts offer less resistance and will flow more air than flex ducts. But in practice, you can size a flex duct larger to get the correct airflow, but it's impossible to seal a hard duct system and keep it sealed. If it doesn't leak when it's installed, it's going to develop leaks over time, which will perform worse than old-but-intact flex duct." His logic is that the cold/hot cycles the pipes see (cold when conditioned air flows, then hot when air stops and pipes warm to attic temp, then cold when air flows, etc., etc.) will stress whatever sealant material you use, and eventually crack it. Repairing seals are costly because you have to remove the insulation and inspect to find them.

    In contrast, he says, flex ducts are stay sealed better. Ensure they're well-sealed at the supply plenum and at the room register, and you know they're intact in-between. Sure, they'll wear out eventually (although all three contractors agree my ducts are in pretty good shape for being 25 yr old builder-grade cheapy stuff), but he points out that you can replace a good flex duct network three or four times for the initial cost of a good hard-duct network.

    I expected my decision to be based solely on cost--could I afford the better hard-duct system, or would I have to settle for flex.

    Is that still the decision I should make, or does this guy have a point that properly-installed flex can outperform properly-installed hard duct over time due to leaks?

    Thanks,
    Jim

  2. #2
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    Flex done right will deliver te same air flow.

    Hard duct done right will not leak over time.He'soff base there.

    It's easier to see the flex was done right and easier to do it right.

    We now have 30 year old flex systems that are in fine shape,no reason to spend the extra for hard duct in my opinion.Many will disagree.

  3. #3
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    Quote Originally Posted by dash View Post
    Flex done right will deliver te same air flow.

    Hard duct done right will not leak over time.He'soff base there.

    It's easier to see the flex was done right and easier to do it right.

    We now have 30 year old flex systems that are in fine shape,no reason to spend the extra for hard duct in my opinion.Many will disagree.
    TOTALLY AGREE!
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  4. #4
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    And ...

    Metal ducting will transmit sound and vibrations that flex won't.

    Go with the flex - done right.
    Do not attempt vast projects with
    half vast experience and ideas.
    ...

  5. #5
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    THe biggest problem I have seen with flex duct is people don't know how to use their duct calculators with it. They grab there regular slides and wheels, most not realizing that they are for metal ducts! They make calculators for flex duct. Flex applied right and sized right will be just fine. Our average leak rate with a sheet metal trunk line and flex duct branch lines is 9-18 cfm, on a 2500 to 3000 sq ft home. Flex can be tighter and will stay tighter( most of the time) Not to say metal is bad, just more labor intensive to be tight. We do both systems. Always use a mastic brush on water based sealant, tapes will not last, and in OR we can't use tapes if we want to qualify the system for energy effcient incentives. Have your contractor do duct blaster after the installation to ensure the duct is tight
    Genius = The guy who can do anything...except make a living!

  6. #6
    Join Date
    Feb 2004
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    here we go again

    flex is produced and installed or one reason. it's cheaper and require less skill to install. we order flex by the truckload and install it in 5' sections max in commercial applications. resi in Chicago is all hard duct. if you can afford the hard pipe do it. unless sized properly neither have a chance of working and that will be your next challenge. finding somebody who knows what there doing in the resi field. unfortunately lots of disappointments there. good luck with that.
    FILL OUT YOUR PROFILE!!

  7. #7
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    Quote Originally Posted by MysticCobra View Post
    I am considering bids from three contractors regarding complete replacement of my A/C and heating in Houston, TX townhouse.

    The inside equipment and ducting will be in the attic, which is typical for this area. My current ducts are 25-yr-old gray plastic-wrapped flex, and I'm including their replacement in my bid requests.

    One contractor insists on installing R8 insulated hard duct, with all joints throughout the system sealed with "Uni-Mastic 181".
    My approach:
    Use 2" ( R-8.3 ) rigid insulation board for 20 to 30 feet as the main Supply Air and perhaps 10 to 15 feet for the Return Air ducts.

    http://www.knaufusa.com/products/com...ion_board.aspx

    30 feet of insulation board would likely be a few/several x00$ premium to the flex duct system.

    For example, 4-ton system with 1,400 CFM would use 18" x 14" main lines for supply air and 22" x 16" for Return Air.

    The branches lines with < 400 CFM can be flexible duct.
    The largest flexible duct diameter would be 10" with maximum length of 15 feet ( or 12" diameter).

    http://www.atcoflex.com/faqs1.html

    Manual D duct design layout should be presented prior to signing of the dotted line.
    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

  8. #8
    Join Date
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    Either duct system will work well if installed properly the key word is installed properly.

    Very few in Houston still do metal ducts and even fewer do it well and even fewer do flex duct well.

    Townhouses are difficult to do properly because much of the ductwork is hidden in the walls and between floors.

    I still do metal duct and always prefer it over flex. When I do use flex I try very hard to keep the length of the runs to less than 8'. The problem with flex in a townhouse is the length of he runs and the ducts that run through chases inside walls and between floors if they can be replaced increasing their size is next to impossible. In addition when installing R-8 flex where space is limited the insulation will be compressed reducing efficiency or the ID of the duct will need to be reduced to fit in the chase.

    Many townhomes in Houston have duct board trunks in between floors or down chases so all you are replacing is the ducts in the attic. These duct run are usaully short and flex will work fine if properly sized and installed.

  9. #9
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    There is no way I'd ever have flex again. It's totally unforgiving to less then perfect installation quality. When it fails whole sections can fall off the junction boxes unless they are perfectly secure. When that happens you can have a PIA job reconnecting them.

    I realize tin also needs to be done right but I don't see how whole sections can fall off over time.

  10. #10
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    What's a flex system? A plenum with supply leaders run to each boot? Is that really a system? I call it an octopus. I've never seen a real flex duct system. That is, as system where the main trunk is flex with proper reductions and branch supplies properly sized. Is that really done somewhere? Whenever space allows, which is most of the time, we install a main trunk, properly sized and thoroughly sealed, metal take-offs and flex duct run STRAIGHT OUT to the supply boots, with our company policy being a maximum length of 15-feet. Longer lengths are metal to within at least 15-feet of the boot.

    We've had experience with many, many clients who've had a system with just a plenum and flex all over the place. They think it works just fine until they experience a properly done system (at least what we call a properly done system). I'm reasonably sure that in Florida they all do flex because they metal rusts out. I'm not sure if that's code or not. I don't know the status of ductboard in Florida or anywhere and I'm not a fan of ductboard either but that's just my personal prejudice.

    What I do know is that if the flex isn't thoroughly stretched tight, supported to be straight without dips and few or no bends in it, then it's not installed properly and airflow will suffer. That's the main problem with flex. It's tempting to cut corners and 95% of the installers out there will cut corners where they can, despite all the best intentions of the owner or specifier. It's much more difficult to cheat w/metal. You can see the mastic before the insulation goes on and you can see the insulation after it's installed. It shouldn't leak and neither should the boots or elbows or plenums anywhere in the system.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  11. #11
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    Metal duct won't rust in the attic.

    Flex is common almost exculsive,though some use ductboard trunks.

    Older homes have metal or ductboard systems.

    The problem with metal in Florida is ,if the wrap is not installed without compressing and vapor sealed seams,it will sweat.Particularly unts like Infinity,that can run at 280 cfms per ton.

    Typical flex system is detailed in Manual D.

  12. #12
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    Thanks for the clarification Dash.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  13. #13
    I should have included some more detail about my townhouse design. Most of the ductwork is in the attic, to supply registers on the 2nd floor ceiling. I only have two downstairs supply registers, and those are fed by a single flex duct running down the same shaft that return air comes through (return air flows around the flex duct in a 2' x 2' sheetrocked chase) (the branch is at the end of the line, just 2' from the registers, which are close physically but in different rooms). The one place I really wish had hard ducts is that supply line down the return chase to the downstairs registers, and that's the one place I can't install hard ducting now without significant sheetrock excavation because the chase is too small to work in. All three bids include new ducts to the point the existing supply flex enters the chase from the attic, and keeping the existing flex from there to the downstairs supply registers.

    One of the two "flex contractors" both recommend a large, custom fabricated plenum with a direct flex line to each supply register (six lines total from the plenum). Due to the small footprint of my townhouse, the two longest runs will be ~20', and the rest will be 15' or less.

    The other flex guy plans to fab one remote plenum to feed three registers. The hard duct guy also plans to use one remote plenum to feed those same three registers. This one also bragged about suspending the ducts from the roof, supported every 4' to prevent sagging. He also talked about how the duct is "built" during the install from a core duct, which is enveloped by an insulation layer, and finished with a vapor barrier outer layer. He talked about the right way to tuck everything together at each end of the run and clamp it off to the collar. Unfortunately, I don't know the right way to install flex duct, but this guy sure sounded proud of the job his guys do. (His bid price reflects that as well, with the ductwork being nearly triple the price of the other flex guy.)

    I think that if money were no object, I would choose to go with metal ducts, and I'll wait to see the bids before I make my choice. But based on the responses here, I think it's likely I'll end up choosing one of the flex duct guys.

    Thanks!
    Jim

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