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Thread: Calculated Flow value does not update, or re-calculate

  1. #1
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    Default Calculated Flow value does not update, or re-calculate

    I have created a VAV system.

    My vav box downstream connectors (multiple duct taps) are set to "calculated" and they dump their calculated values into instance parameters. The upstream connector is preset and sums the downstream connector parameters.

    When I first placed these families everything worked great. The flows specified at the diffuser where propagated to upstream the vav box. But when I went back and changed the flow value at the diffuser, the flow upstream the vav box remained the same. The parameters which initially received the "calculated" value will not update. I tried saving and reopening the project, and disconnecting and reconnecting the duct but it will not recalculate.

    Any ideas?

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    Default Re: Calculated Flow value does not update, or re-calculate

    Check what VAV type you are using and its parameters and how it is pulling the information. It may be at the VAV that is the problem. it could be using a calculated value ir a percentage of the values or something crazy like that. Is the VAV unit a self made unit or OOTB? Can you post the unit for more investigation?

  3. #3
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    Default Re: Calculated Flow value does not update, or re-calculate

    it was OOTB. it did not function at all until I modified the parameters. After modifying, it calculates the value once, but does not re-calculate.
    i'm not sure what u mean by "vav type"?

    Family : "VAV Outlet Plenum - Tee" 2013

    Connector 1
    Flow Preset
    Direction In
    Supply Air
    Specific Loss
    Flow "PrimaryAirflow" parameter (PrimaryAirflow = SupplyAirflow + SupplyAirflow2)

    Connector 2
    Flow Calculated
    Direction Out
    Supply Air
    Loss Not Defined
    Flow "SupplyAirflow" parameter

    Connector 2
    Flow Calculated
    Direction Out
    Supply Air
    Loss Not Defined
    Flow "SupplyAirflow2" parameter

  4. #4
    Revit Forum Manager Steve_Stafford's Avatar
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    Default Re: Calculated Flow value does not update, or re-calculate

    The relationship between components is Child "looking up" to Parent. That means that air terminals "look up" for their parent VAV (or source of air). The VAV then looks for its parent AHU. Most common the Air Terminals are assigned to preset, this means that we provide a desired flow at the terminal. The receiving connector at the VAV is assigned to Calculated, which means it calculates the air flow defined by the air terminals that are preset to specific values. In this way four air terminals assigned 150 CFM will contribute 600 CFM to the VAV.

    At the VAV the Air Flow that it passes on to the AHU is usually assigned in the same fashion. The total flow that it receives from the air terminals is NOT automatically passed along to Air Flow parameter. As the engineer you decide what VAV is the appropriate size once you see what the total load on it is. That means it may be expected to deliver 600 CFM based on the previous paragraph but you as the engineer may choose to use a unit that can deal with 1000 CFM instead. This 1000 CFM is the Air Flow you'd provide to the VAV to pass on the AHU. The AHU would then "see" all the CFM requirements of VAV units connected to it and the engineering choice would repeat to decide how big the AHU should be.

    If you make sure those assumptions are true of your connected network they should work. Start at the air terminals and work "up" to the VAV and make sure you get the total CFM there that you expect. Then work up to the AHU. You can use the System Browser to examine the "tree" relationship the components have. You can also use Check Duct Systems (Analyze tab) to see if Revit can generate warnings to give you clues what its missing.

  5. #5
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    Default Re: Calculated Flow value does not update, or re-calculate

    i went back into the project. the problem with it not re-calculating is inconsistent. I created a new instance and it worked fine. the original boxes which would not re-calculate did after I set them to 0. So... i don't understand what the problem was. After dealing with the flashing re-host button for loaded face based electrical panels I feel like chalking it up to bugs...

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