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Thread: "Air Flow" parameter

  1. #1
    Senior Member Ning Zhou's Avatar
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    "Air Flow" parameter

    most OOTB air-related Mechanical Equipment have "Air Flow" instance parameter, when loaded into project, some are read-only, any logic and how to change it?
    Attached Thumbnails Attached Thumbnails "Air Flow" parameter-air-flow.png  

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    I'm looking at those OOTB families and don't see any greyed out air flow parameters. You should be able add/delete parameters at will. I also don't see any Shared Parameters in this family and you're going to need them for scheduling and/or tagging.

    I've built a mature library of MEP components over the years and I didn't know that there were so many OOTB MEP families. When I started with Revit back in 2011 it didn't come with this much stuff. I've opened a few of them just to poke around and I have to say that I'm not impressed. These weren't made by people who do MEP engineering work. For instance that coil family has this formula in its electrical Apparent Load parameter:

    65 W/ft² * Duct Width * Duct Height.

    Who the hell sizes a duct heating coil based on watts per square foot of duct cross section? No engineer anywhere would size a duct coil in this way. And even if you wanted to do it this way, why would you write that '65 w/ft2' value when you could use a parameter that could be edited? Meanwhile there are no parameters for Coil KW, entering air temp or leaving air temp, which (I think...) are fairly important things to know about your electric duct heating coil. This family is junk. Use these OOTB families with caution. You're probably better off making your own from scratch TBH. By the time you're done tinkering with the OOTB family you could have made your own and a) gained skill and b) gotten exactly what you want.
    Last edited by Necro99; January 4th, 2019 at 09:58 PM.

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    Senior Member Ning Zhou's Avatar
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    thanks Necro99, see uploaded RVT (created w/ no template and loaded OOTB family) for your info., i won't use that family in real project, but i just wonder what's causing read-only and how to change it, my workaround is creating new parameter say "my Air Flow" and make OOTB "Air Flow" equal to that, well, just curious what's going on?!
    have great weekend!
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    Member kubsix's Avatar
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    The family's connector(s) determines the state of the Air-Flow parameter. Setting the conector's Flow Configuration parameter to Preset makes it editable.

    Systems use a Preset connector as the user input value at the end/source devices and wherever a Preset connector is a new System is defined. That's why the VAV's inlet is set to Preset as it is the source for the AHU. Whereas an Air Terminal and it's VAV box is one system, the VAV box and the AHU is another. The VAV box is configured to pass the CFM value between its two connectors via the air flow parameter. Values will no longer carry through if you change it at the VAV box. I am trying to understand how to reset that. However it is often desirable to have VAV's airflow value (what the AHU sees) less then the sum of the Air Terminal's (the difference between sum of zone peaks and AHU block load). So it is good to be able to change it at the VAV box.

    The duct coil does not interrupt or change the airflow like a VAV box does, and it should just pass the CFM value through. The air terminal, duct coil, and AHU are all one system in this case. I do think that family is jacked up and does not work properly as-is. It's Air Flow value should be set to 0 for one and not 800. It might also be better as a duct accessory instead of mechanical equipment.


    https://thebuildingcoder.typepad.com...ep_content.pdf
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    Forum Co-Founder Alfredo Medina's Avatar
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    Quote Originally Posted by Ning Zhou View Post
    ..., but i just wonder what's causing read-only and how to change it...
    It depends on the value of the Flow Configuration parameter in the duct connectors in the family. The options are Preset, Calculated, and System. This family has that setting as Calculated and the connector is associated to a parameter that contains a value. That's why the Flow parameter is read-only for you in the project. To make it editable yo would need to open the family and change the flow configuration setting of the connectors.

    See attached PDF by Martin Schmid, P.E. about this topic.
    Attached Files Attached Files
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    Forum Addict josephpeel's Avatar
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    Quote Originally Posted by Necro99 View Post
    No engineer anywhere would size a duct coil in this way.
    Yeah, this is the fundamental problem with families for MEP. The software providers dont know enough about engineering to make something that works and not many engineers know enough about the software to make good families.
    Even the big providers of MEP content such as MagiCAD, CADAC, Stabiplan etc. cant make families that fit into the specific workflows or requirements of particular companies or projects. So having someone with engineering knowledge who can also make families that work, as part of a wider design process, is vital. Not having someone who can bridge that software/engineering gap or not making time/budget available to do it is where many companies seem to fall down with implementing MEP Design in BIM.

    There is so much that could be done in a smarter or more automated way but isnt because design methods/processes and the tools/content available dont quite match up as a working whole.
    Last edited by josephpeel; January 6th, 2019 at 08:11 PM.
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    Quote Originally Posted by kubsix View Post
    and wherever a Preset connector is a new System is defined.
    That's good to know! I've observed this behavior and wasn't entirely sure what was driving it. I recently made a DYN that re-ames Duct Systems based on the Mark of the Mechanical Equipment in each System and things got tricky when I realized that each VAV box was actually part of two systems. I got it to work for VAVs, FCUs, and ducted AC units, but any "in-line" equipment like in-line fans or duct heating coils will foul it up because there's an upstream system and a downstream system.

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    Quote Originally Posted by josephpeel View Post
    Yeah, this is the fundamental problem with families for MEP. The software providers dont know enough about engineering to make something that works and not many engineers know enough about the software to make good families... Not having someone who can bridge that software/engineering gap or not making time/budget available to do it is where many companies seem to fall down with implementing MEP Design in BIM.

    There is so much that could be done in a smarter or more automated way but isnt because design methods/processes and the tools/content available dont quite match up as a working whole.
    Addressing this is the direction that I want to take my career in, but I'm not sure how make it happen.

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    Member kubsix's Avatar
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    Quote Originally Posted by Necro99 View Post
    ... but any "in-line" equipment like in-line fans or duct heating coils will foul it up because there's an upstream system and a downstream system.
    It appears that if the connectors are "linked" the system will be the same on both sides. Won't change systems for families already place.

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    Quote Originally Posted by kubsix View Post
    It appears that if the connectors are "linked" the system will be the same on both sides. Won't change systems for families already place.
    Hey, thank you so much for this! I never really knew what linked connectors were for but now I do! The connectors also seem to need to be set to "Global" and not to a specific System Classification.

    According to this, "Linked connectors only have an effect when the System Type is set to Global, which is common for fittings and in‐line components such as dampers, valves, and pumps."

    https://knowledge.autodesk.com/support/revit-products/learn-explore/caas/CloudHelp/cloudhelp/2015/ENU/Revit-Customize/files/GUID-ED50E6C8-F71C-4AF8-AB4B-113A93C3F541-htm.html

    The more I think about it, the more I think that my Duct Heating Coil family should be a Duct Accessory and not Mechanical Equipment. It would make my System Naming DYN work much more smoothly.
    Last edited by Necro99; February 15th, 2019 at 01:15 PM.
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