sizing ductwork
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Thread: sizing ductwork

  1. #1
    I want to kno a easy way to figure out what size duct board to run on an install. this is how I was taught please set mme straight.

    1. start with the size of the opening above the furnace,( on a typical updraft unit say 3 ton
    2. make that extend into ceiling .
    3. then make trunk duct 12-14 square
    4 take offs are 7" take off with flex for 800-1000 squre feet and adjust acordig to squre feet and heat load from there.
    I am a good refrigeration tech and a/c troubleshooter but need help with duct work. can some one help me out.

  2. #2
    Join Date
    Mar 2004
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    You need help. I don't think any of us on here can help you over the internet, we can start by saying that is not the correct way to do it though, but is a hack way to do it. You must use a ductulater.

    Your best bet would to go to a vo-tech and get some training from them.

    Btw.........welcome and enjoy your stay, you will learn a lot here, if you ignore the BS. Outside rep has a BS meter, you can ask him for it.
    Karst means cave. So, I search for caves.

  3. #3
    I can get a ductulator at my local supply house If I know my total square footage . how can I find the size duct leavig my furnace does it become different tan my furnace opening

  4. #4
    Join Date
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    Like they told you on the other thread you need to do a load calc first. This will tell you total cfm and cfm to each room. You then use your ductulator to find out size of trunk line. Your plenum will depend on furnace opening and trunk design. You maybe in over your head on starting an install buisness. You may want to start with service and sub out your installs until you learn the install side.
    Karst means cave. So, I search for caves.

  5. #5
    Join Date
    Apr 2004
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    Bartlett, IL
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    Thumbs down

    What the hell is ductboard? gotta use METAL!

    I can't even understand your question so I'm outta here!

  6. #6
    Join Date
    May 2004
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    Rapid City, SD
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    7,414
    Keep in mind duct board has a different friction rate than metal also.... aka different duct-u-lators.
    "If you call that hard work, a koala’s life would look heroic."

  7. #7
    so if I stay with my opening off my furnace close my plenim off at the top use my ductulator to determine my trunk size, and use my formulas in my a/c refrigeration book to determine my heat load for my takeoffs I will be in good shape.

  8. #8
    I have only used ductboard never sheet metal seems like metal is herder to work with

  9. #9
    Join Date
    Mar 2004
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    Originally posted by biloxitech
    so if I stay with my opening off my furnace close my plenim off at the top use my ductulator to determine my trunk size, and use my formulas in my a/c refrigeration book to determine my heat load for my takeoffs I will be in good shape.
    I doubt it. Expect call backs.
    Karst means cave. So, I search for caves.

  10. #10
    Join Date
    Sep 2004
    Posts
    3,157
    The first thing you need to do is figure the required load for the house . we have a nifty little program at the top of the page (the red button) that will assist you in determining the proper size equipment for the area you are in and being in Biloxi you will have that humidity to consider. to undersize your equipment or to oversize your equipment will result in some very uncomfortable conditions

    once you do the hvac calc then you can determine the cfm to each room
    Naturally the plenum starts at the size of the opening of the furnace but from there the size may need to increase
    the flex that you run may only need to be 6"

    If you have a local community college that has an hvac program that survived the hurricane, check with them

  11. #11
    Join Date
    Mar 2004
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    SE Michigan
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    If you have a local community college that has an hvac program that survived the hurricane, check with them


    "Politicians are the lowest form of life on Earth. Liberal Democrats are the lowest form of politician"

    - General George S. Patton

  12. #12
    Join Date
    May 2003
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    Dixie
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    MGCCC..Jeff Davis campus has an excellent program....and they are up and running .. not much damage

  13. #13
    Join Date
    Jul 2005
    Location
    South Carolina
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    351
    They were pushing ductboard at the local duct fair back in October. Nothing but problems. always coming off and if it ever gets wet!!

    Get a sheet metal plenim!
    Philippians 4:13

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