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How do you account for diversity when sizing ductwork mains in a VAV system?

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    How do you account for diversity when sizing ductwork mains in a VAV system?

    I've seen people use flow factor to reduce the flow in an airhandler and the main duct off of it from the sum of the peaks of zones to the block load, but that has been giving me weird results. Flow Factor obviously wasn't made for that.

    I've been taught that the main ductwork should be sized for the block load until the sum of peaks flow is less than the block. I've been sizing the mains by ductulator until that point, but would like to know if Revit has a better way.

    Watered down version: using a combination of velocity and pressure loss i.e. not to exceed max velocity OR max pressure loss. Other factors might include what space type the ductwork is passing over to be cognizant of breakout noise.

    Edit: Although Revit provides the ability to set rules for the above, I've found it to get a little wonky at times so you'll have to be sure to check the results.
    Last edited by chompi0n; April 28, 2017, 02:53 PM.


      For diversity, I would suggest making a Duct Accessory with connections on both sides, that reduces the flow through it by the diversity factor.
      Or another method (That I use a lot) is to just divide your system up at the locations where diversity can be applied and use an 'endcap' that has a flow rate to set the flow from that point. For example where the ducts come out of a riser, place an endcap with the max diversified flow for that floor. Then create a separate system on the floor itself. (This also makes it easier to copy whole floors).
      "One must imagine Sisyphus happy." Albert Camus - "The innovator has for enemies all those who have done well under the old conditions, and lukewarm defenders in those who may ​do well under the new." Nicolo Machiavelli -"Things that are too complex are not useful, Things that are useful are simple." Mikhail Kalashnikov


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