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Thread: Duct connector causing formula to allow wrong answer?

  1. #1
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    February 1, 2019
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    Angry Duct connector causing formula to allow wrong answer?

    Hi y’all, first time posting because I haven’t been able to find any other posts with this same issue, only the opposite of my issue?

    I built a Mechanical Equipment family for a terminal unit that auto-sizes based on scheduled airflow. The size of the terminal unit was driven off of an instance-based length parameter, Unit Size. Unit Size has a formula in it with nested if statements, comparing a scheduled airflow to preset maximums for each size (6” - 350 cfm, 8” - 700 cfm, etc.). This works great and allows me to fill in my sizing criteria and let each box decide what size it needs to be for each instance. And gets rid of the need for types to drive unit size.

    The only problem is, when I assign that parameter to the size of a duct connector in the family, I found that connecting a different size piece of duct to that connector will change the parameter length to match the duct. This changes the scheduled size of the terminal unit, and allows the parameter to equal a value that is not equal to the formula result.
    For example:
    Scheduled airflow = 400 cfm
    400 cfm > 6” max, but 400 cfm < 8” max
    Unit Size = 8” (according to the formula)
    If I draw a 12" duct into the duct connector, the Unit Size becomes 12". Likewise with any other size I connect into the duct connector. Since the Unit Size parameter drives everything else in the family, this changes what size box I have scheduled completely, even though the formula should output 8".

    I haven't been able to come up with a work-around to force the Unit Size to only equal the result of the formula. If I set up airflow to a different value, and back to 400 cfm, it will fix itself and add a transition at the duct, but this concerns me. This could severely throw off our terminal unit schedules.

    I really want the Unit Size to be instance based, to eliminate the need for types driving the size. Is my problem just a result of the parameter being instance-based, or am I missing something?
    Any thoughts? Has anyone else attempted this and run into the same issue?


  2. #2
    Forum Co-Founder Alfredo Medina's Avatar
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    December 7, 2010
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    Welcome to the forum.

    Interesting question/problem. I think this could be solved with a lookup table. That means that you would need to abandon the idea of keeping the dimensions of the family as instance-based. In the lookup table, you can specify the values of all the type parameters for every different size of a duct.

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