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  1. #1

    Sheet metal ducts Vs. Flex ducts

    First let me start by saying I am not in the HVAC business. I am the owner of a home built in the 50's. We have the original galvanized sheet metal ducts in our basement for the HVAC. It was suggested (by a moisture control co.) to have the ducts looked at--some needed to be repaired. So far, we have gotten 4 estimates--2 companies say we need to tear out all the old ducts because they are metal and the seams will always leak. The other 2 co. say that the old metal ducts are the best there is for air flow and not to take them out. They suggested sealing the seams with mastic. Is there a benefit of 1 type of duct over the other? We have not had any problems w/ our system (the house heats and cools efficiently). The co. only found one duct that needs repair. Any suggestions would be greatly appreciated!

  2. #2
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    Sep 2006
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    Missouri
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    sounds like sealing with mastic is perfect for this job.especially if only one duct is damaged unless you are having allergy or dust problems.if not why tear it out if its working.

  3. #3
    Join Date
    Dec 2006
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    Salem
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    They are right, hard pipe ducting is best for airflow but in the old days, they did not seal them (seams). Flexable ducting has ribbed coils in the tubing which is not the best airflow but is alright at the same time. Stick with metal piping. They have an HVAC sealing tape called "BUTAL" or 367 tape made by NASHUA Its a little spendy it works very well. Buy a few rolls of that and run a few zip in screws in your down duct and seal it with that tape and it will be good to go for a very very long time. Do not use duct tape, its out of code and flakes off over time... Hope that helps...

  4. #4
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    Jul 2004
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    A recent study indicates that the flow restrictions with flex duct are very substantially higher than even the most conservative tables in use. Sheetmetal ducts are ALWAYS better for airflow purposes. We install sheetmetal all the time, restricting our installers to a maximum of 15-foot supply branch runs of S-T-R-E-T-C-H-E-D flex ducts. Flex can be good to help dampen velocity noises and it is relatively air tight but IMO, mastic on the old sheetmetal ducts, covered by a good foil backed insulation is without question the best way to go, old ducts or not. Throw the flex guys out as bottom feeders. Keep what you've got and upgrade it with mastic.
    If YOU want change, YOU have to first change.

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  5. #5
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    both have there place in the industrie no mater the restriction rate

    in the fifties most of the metal ducts were rectangler duct 10x3 1/4 or 12x3 1/4

    rectanguler duct is the least restrictive of all because air travels in a circulor motion and forms little ball bearings of air pockets in the corners creating the perfect smooth surface for the air to flo through.

    round metal pipe is next in line even tho it is round it has restriction from the hard surface of the pipe itself

    flex is the most restrictive because of the wire ripples in it causing the air to bouce arround inside more

    hope this helps you understand the differance for your dicition

    keep in mind that the duct work from the fifties is in most cases was not sized large enough for todays ac systems. Even tho back then most furnaces were way to big for the house because no one cared about cost just heat.
    the ducts were designed to move air with a minamal amount of blower cfm

    your ducts need to be sized to make sure they can handle the ammount of cfms you will need for ac and then sealed up and resaported.

    if none of the companies sized your system and ducts then IMO they are all wrong

  6. #6
    Join Date
    Aug 2007
    Location
    Maryville, Tennessee
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    12
    First,don't try to fix that what ain't broke.Seal all that needs sealed.Is the insulation in good shape?Do what is needed to get the insulation in good repair.That should clear up moisture issues.I don't think that there is a thing wrong with some flex in a duct system when installed properly.I use a little flex on the end of most all my supply runs and I'd still be out there tying in package units without the use of flex.Key is proper installation of the flexI believe.

  7. #7
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    Stay with the metal

  8. #8
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    Quote Originally Posted by tinknocker service tech View Post
    both have there place in the industrie no mater the restriction rate

    in the fifties most of the metal ducts were rectangler duct 10x3 1/4 or 12x3 1/4

    rectanguler duct is the least restrictive of all because air travels in a circulor motion and forms little ball bearings of air pockets in the corners creating the perfect smooth surface for the air to flo through.

    round metal pipe is next in line even tho it is round it has restriction from the hard surface of the pipe itself

    flex is the most restrictive because of the wire ripples in it causing the air to bouce arround inside more

    hope this helps you understand the differance for your dicition

    keep in mind that the duct work from the fifties is in most cases was not sized large enough for todays ac systems. Even tho back then most furnaces were way to big for the house because no one cared about cost just heat.
    the ducts were designed to move air with a minamal amount of blower cfm

    your ducts need to be sized to make sure they can handle the ammount of cfms you will need for ac and then sealed up and resaported.

    if none of the companies sized your system and ducts then IMO they are all wrong
    This is the first time I recall reading a duct with a higher aspect ratio having better air flow .

    I always thought the lower the aspect ratio and closer to round you got the more air would pass though a duct. Or as the ducts aspect ratio got lower you could pass the same amount of air though a smaller duct.


    Example: Set a duct calculator @
    800 CFM
    700 FPM
    .06" friction rate

    You get:
    22" x 8" = 176 sqin of duct
    13" x 13" = 169 sqin of duct
    14" round = 153 sqin of duct

    The ducts sqin gets smaller but moves the same amount of air.



    My first choice would be a system that is sized correctly.

    And then would chose metal over flex .
    Ed J

  9. #9
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    SC
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    If your ductwork is properly sized, it will be better than flex. But it needs to be sealed and insulated to today's standards.

  10. #10
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    Aug 2003
    Location
    Fort Worth, TX
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    Quote Originally Posted by Ed Janowiak View Post
    This is the first time I recall reading a duct with a higher aspect ratio having better air flow .

    I always thought the lower the aspect ratio and closer to round you got the more air would pass though a duct. Or as the ducts aspect ratio got lower you could pass the same amount of air though a smaller duct.


    Example: Set a duct calculator @
    800 CFM
    700 FPM
    .06" friction rate

    You get:
    22" x 8" = 176 sqin of duct
    13" x 13" = 169 sqin of duct
    14" round = 153 sqin of duct

    The ducts sqin gets smaller but moves the same amount of air.



    My first choice would be a system that is sized correctly.

    And then would chose metal over flex .
    You are correct. I'm not sure where tinknocker got this idea:

    rectanguler duct is the least restrictive of all because air travels in a circulor motion and forms little ball bearings of air pockets in the corners creating the perfect smooth surface for the air to flo through.
    What he may not realize that if his theory is even partly true; re: aerodynamic "ball bearings", it would take energy to form and sustain those "ball bearings", and the presence of these "ball bearings" would create turbulence further into the airstream than he might think.

    Airflow through ducts is seldom laminar. It can be at lower velocities, but at the velocities often needed for resi/light commercial work, any time air is moving through the duct it will have pockets of turbulence that add to friction loss through the duct.

  11. #11

    Thanks

    Thanks for all the advice. I 'll make sure that the system is sized correctly and go from there. Again, thanks for the help!

  12. #12
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    Quote Originally Posted by shophound View Post
    You are correct. I'm not sure where tinknocker got this idea:



    What he may not realize that if his theory is even partly true; re: aerodynamic "ball bearings", it would take energy to form and sustain those "ball bearings", and the presence of these "ball bearings" would create turbulence further into the airstream than he might think.

    Airflow through ducts is seldom laminar. It can be at lower velocities, but at the velocities often needed for resi/light commercial work, any time air is moving through the duct it will have pockets of turbulence that add to friction loss through the duct.
    Smacma standarts. Is where i have this from. The book tells you everything about airflo and will show you what will happen with the air inside the duct. It also will show you what most applacations will do to the air as far as turbulants. Turning vanes and elbows, differant takeoffs and so on

    we all know with resi and light com we have learned to cut corners and use radious 90 with square throats witch are a big cause of turbulants also have seen turning vanes with radious 90 witch is even worst. Turning vanes is well explained in the Smacma book

    very informative when it comes to airflo and turbulants

    my comment was about air flo resistance not static pressure. I have used this book for years and find it to be very helpfull

  13. #13
    Join Date
    Jul 2005
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    5,153
    Quote Originally Posted by tinknocker service tech View Post
    Smacma standarts. Is where i have this from. The book tells you everything about airflo and will show you what will happen with the air inside the duct. It also will show you what most applacations will do to the air as far as turbulants. Turning vanes and elbows, differant takeoffs and so on

    we all know with resi and light com we have learned to cut corners and use radious 90 with square throats witch are a big cause of turbulants also have seen turning vanes with radious 90 witch is even worst. Turning vanes is well explained in the Smacma book

    very informative when it comes to airflo and turbulants

    my comment was about air flo resistance not static pressure. I have used this book for years and find it to be very helpfull
    You can't separate static pressure and resistance in a duct system, they're flip sides of the same coin.

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