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Thread: Duct Sizing

  1. #1
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    Duct Sizing

    Just curious, what is the proper way to size the main trunkline using ductboard?
    After using a duculator to find proper size should you start with the size of the throat and transition up? Or just start with the calculated size wich is larger than the throat?

  2. #2
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    First step, get a Manual D.

    STUD

  3. #3
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    Yeah well im just the tech thats replacing an a/h and some ductwork. Never got any dimensions

  4. #4
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    You should just be able to transition from the size of the supply from the furnace to the size of the trunk. I would stay away from duct board. Your cust will appreciate it later on.

  5. #5
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    Quote Originally Posted by SgtBone View Post
    You should just be able to transition from the size of the supply from the furnace to the size of the trunk. I would stay away from duct board. Your cust will appreciate it later on.
    If duct board is properly installed the is nothing wrong with it and has its advantages.

  6. #6
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    Quote Originally Posted by second opinion View Post
    If duct board is properly installed the is nothing wrong with it and has its advantages.
    +1 Emphasis on properly.

  7. #7
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    I guess ive never seen it properly installed.

  8. #8
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    Quote Originally Posted by ka24det View Post
    Just curious, what is the proper way to size the main trunkline using ductboard?
    After using a duculator to find proper size should you start with the size of the throat and transition up? Or just start with the calculated size wich is larger than the throat?
    Good rule of thumb is .06 ft/100 on a ductulator. Tranistion up if the outlet from the unit is smaller. Cant really be to big within reason. Nothing wrong with properly sized/installed ductboard in a residence.

  9. #9
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    Like a lotta things, the answer is "it depends"

    Quote Originally Posted by ka24det View Post
    Yeah well im just the tech thats replacing an a/h and some ductwork. Never got any dimensions
    First question is: Is it a big fat plenum or a high velocity discharge? think of the air as a fire hose, is the water being put into a 55 gal drum or a bucket?
    the high velocity discharge needs gentle size transitions OR the high velocity air gets turbulent and the velocity doesn't convert to static pressure. It's called system effect losses

    The other thing to remember is that the choice of sheet metal or ductboard ( people think the extra resistance of board is a big deal) is not much different in air delivery because most of your duct losses are the fittings!
    In fact the extra resistance of ductboard is more than offset with it's really low leakage Ductboard is expected to be 1/8 the leakage of sealed residential duct

    That's right Boys and Girls, leak class 6 vs. leak class 48 NOT A Typo

    So the real advice is to think about the discharge velocity for your immediate problem of doing a good tie in and if the customer is honest and tells you they have air delivery problems now, your new box even with a 1 hp ECM motor is NOT GOING TO FIX IT Now you need to really understand what is hanging off the box and figure out the fix
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  10. #10
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    I do agree there is nothing wrong with ductboard as long as it is assembled properly, and genduct its a res 2.5 ton heil air handler with an x-13 ecm motor. So not exactly sure if its high velocity or not? But judging from your analogy i would say high velocity.

  11. #11
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    So a transition from the supply of the a/h to the proper calculated size is right??

  12. #12
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    A high velocity discharge

    would be less than one sq fi discharge area more than 1000 FPM
    The best thing is to make a real transition that would gently open from the discharge size to the duct size you need with no more than 15 degree angles 1 inch to 6 OR just know that you are going to take a pressure drop from the stated pressure available for the airflow you need Probably going to lose .1 or .15 at the unit and that's if the return duct is short and sweet

    Beyond that not seeing what you have, it is tough to suggest anymore
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

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