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  1. #1
    Join Date
    Jul 2004
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    322
    Let's say all the ducts come off a single supply plenum. If I have an 8-inch supply duct and replace the existing 8-inch takeoff with a 10-inch takeoff and a reducer, would that increase airflow to that duct?

  2. #2
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    Probably not enough to notice.

  3. #3
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    Using a shortened collar,so the reduer is as close to the plenum as possible,you will reudce the equivalent feet of that run by 25 feet.So if the runs is 175 TEL now,and you reduce it to 150 TEL,,might notice the the diference.

    Guessing that the availaibe static in the plenum is .15,the
    the 8" could be at 200 cfms and go to 230.This is just guessing without knowing the TESP,and seeing the complete duct layout,I'd say you won't get more then this and could easily get less improvement.

  4. #4
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    5,034
    Use the 10" takoff and Run the 10" duct out as far as you can then go the reducer to 8 inches. That "might" (too many unknowns) get it closer to 250-cfm but will increase the velocity through the boot and diffuser.

    http://www.udarrell.com/proper_cfm_b...g_systems.html


  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    Originally posted by udarrell
    Use the 10" takoff and Run the 10" duct out as far as you can then go the reducer to 8 inches. That "might" (too many unknowns) get it closer to 250-cfm but will increase the velocity through the boot and diffuser.

    http://www.udarrell.com/proper_cfm_b...g_systems.html


    The friction rate quoted in your site is not applicable to all duct systems,many need to be designed at .06 or .08,as opposed to .1 or .09.A manual D design will give you the correct FR for every job.

  6. #6
    Join Date
    Jul 2005
    Posts
    717
    Under normal conditions, yes you will increase the volume by at least by 40-50 %. now if the velocity increases approx 15 % along with this added volume , you'll be ahead.
    And i don't think the 'TEL' effect will ever be noticeable (as Panama also stated) by so-called "shortening" of the length of the takeoff.
    Keep the reducer as far downstream as possible.

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by deejoe
    Under normal conditions, yes you will increase the volume by at least by 40-50 %. now if the velocity increases approx 15 % along with this added volume , you'll be ahead.
    And i don't think the 'TEL' effect will ever be noticeable (as Panama also stated) by so-called "shortening" of the length of the takeoff.
    Keep the reducer as far downstream as possible.
    Okay ,now let me understand this,you predict the volume will increse by 40 to 50 %,and Panama agrees by saying "probably not enough to notice".

    Can you tell me what you base the 40 to 50% increase on??

  8. #8
    Join Date
    May 2005
    Location
    Mississippi
    Posts
    102
    Sorry guys, I am with Dash, I just dont get it?

  9. #9
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,517
    i dout that it will make any differance at all. to make any real differance would be to increas the run all the way to the diffuser. the other thing would be to run a trunk line sized for the amount of cfms you need and make all the take offs a lot shorter. a manual d will help with this. most air handlers need a pleum at least or close to three ft long then the trunk connected into it. to have all of the runs off the pleum is just free fall and not much for control of the air.

  10. #10
    Join Date
    Dec 2002
    Location
    in a house, Appomattox, Va.
    Posts
    3,442
    Different location on duct system but....

    while reading through manual d I noticed that large takeoffs w/reducers have a pretty significant effect on airflow in round trunklines.
    Col 3:23


    questions asked, answers received, ignorance abated

  11. #11
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,248
    Originally posted by fat bob
    Let's say all the ducts come off a single supply plenum. If I have an 8-inch supply duct and replace the existing 8-inch takeoff with a 10-inch takeoff and a reducer, would that increase airflow to that duct?
    I would say increase the size of the duct all the way to the boot,or add an additional branch.

    Depends on how much CFM you are short to the room.
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

  12. #12
    Join Date
    Jul 2005
    Posts
    717
    dash; simple math; 8" = 50"
    10" = 78"

  13. #13
    Join Date
    May 2005
    Location
    Mississippi
    Posts
    102
    I understand the idea of thinking, but he is still just running it through a 8" diameter duct from the take-off attached to the plenum.

    I doubt he will notice any increase in airflow worth the effort.



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