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Thread: Fixture Units

  1. #1
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    Question Fixture Units

    Hi guys

    I have a question regarding Fixture Units (FU) of Plumbing fixtures in Revit.

    In my country DIN norms are accepted, and by DIN norms the FU equals 0,25 l/s and it is a fixed value

    I can't find anywhere what is the number of FU in Revit or is it even possible (and how) to change it. I know it is a default Mechanical Parameter in the Plumbing fixture family and that looks like it cannot be modified.

    So I connected a few plumbing fixtures onto domestic cold water system to see the flow and FU sum for that system. Based on those two numbers I calculated FU to be 0,286 l/s.

    That would be fine if I didn't add a few more plumbing fixtures and got different result - 0,278 l/s. So I added a few more and I got 0,266 l/s. Now I'm confused

    Any ideas why is it variable and how to modified it / fix it / change it?
    Last edited by Croat; February 14th, 2014 at 01:37 PM.

  2. #2
    Forum Addict tzframpton's Avatar
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    First, check your own calc's with Revit's source, the "2006 International Plumbing Code (IPC), Table E103.3 (3)" as noted in the Help File:
    Help

    Can you confirm that your calculations are consistent with Revit's calculations?

  3. #3
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    Now that you've mentioned it....... I've remembered that few weeks ago I was looking which formula Revit uses to calculate domestic water flow. I downloaded that table and checked my calculation with the data from it. Everything matched regarding FU and flow. BUT yesterday, when I check with my DIN tables the results does not match. And that is the problem. I get deviations in results.

    I thought that Plumbing Code and DIN use the same formula and that only FU is different, so that I will be able just to alter the FU value. Now I see there is no possibility to do that since REVIT uses fixed results - Plumbing code tables

    I can't find the equation for Plumbing code FU
    Last edited by Croat; February 15th, 2014 at 10:11 AM.

  4. #4
    Member ThatBIMGuy's Avatar
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    Are the connections setup to utilize Fixture Units as the method of flow? PF by default have a WFU, CWFU, and HWFU parameter. You just need to tell the connection to use fixture units instead of calculated or preset, change the system classification to one of those and give it a value. That value will calculate through the pipe.

  5. #5
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    Yes they are. But there is no possibility to change calculation method e.g. calculation formula.

    FU calculations are fixed to the International Plumbing Code and cannot be changed

  6. #6
    Forum Addict josephpeel's Avatar
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    Making pipe calculations with other forumulas;

    Ive had similar problems as I work in the Netherlands, where they use a nominal flow for waste water instead of FU. This method works for any kind of sizing calc (I also use it for sprinklers and gasses and stuff).
    -First draw your pipe system using only placeholders (The reason why you dont want to use pipes will soon become clear..)
    -Connect all your end fixtures with flow rates (Use hydronic return) or FU.
    -Make a pipe placeholder schedule with system type (to filter), flow/fu, pipe size and a calculated value for the sizing calculation using the local formula
    -Sort by Calculated size and turn off Itemise all instances, so you can change lots of sizes at once. Also useful to round calculated size using (round([123.45]/10))*10 = [120]
    -Change the "actual" Pipe Size column to match the calculated sizes, which will directly change the sizes in the model. If you do this with pipes you will get hundreds of errors because the fittings dont change size and revit makes 2x transitions for every connection...
    -Copy your sized placeholder design to another workset so you can come back to it to make changes.
    -Convert all placeholders to pipes. Done!

  7. #7
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    Nice workaround
    Also good example to do things another way

    Thanks!

    I hope that Autodesk will think about us EU guys in future Revit editions and alow FU formula customization
    Last edited by Croat; March 11th, 2014 at 07:26 AM.

  8. #8
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    Hi guys,

    I have the same problem of Croat...
    I work in the UK and here we don't use FU. We use the BS EN 806-6:2006 standard to size the pipes for domestic water (and so we use Loading Units).
    I was searching a solution about my problem and I found this post.

    In my opinion Josephpeel has found a very cleaver solution indeed.
    I tried to use that solution and it works =)

    But I've found a problem.... let me explain maybe with an example.

    I've got 6 WCs
    For each WCs I put 1 Fixture units (So the total Fixture units are 6 FU).




    When I click on a pipe placeholder (1,2,3,4,5,6) the properties panel says that each pipe placeholder is connected just with 1 FU (Which is correct).
    (See pic no. 1)


    The problem is when I click on the main pipe placeholder (A) at high level.
    As it's just one pipe placeholder, it will never have the correct sizes.
    For example, if the main pipe placeholder after 3 FU has to change from ě15 to ě22ě,
    and after 5 FU has to change from ě22mm to ě28mm, with the Josephpeel's suggestion that is not possible because I have just one pipe placeholder at high level.
    Fixture Units-1.pngFixture Units-2.png



    If I want to size the pipe placeholders in a correct way, the main pipe placeholder at high level should be not just one but more than one (A,B,C,D,E,F). (See pic no. 2)


    Because in that way, if I click on the pipe placeholders at high level, the properties panel should tell me:


    pipe placeholder A has => 1 FU
    pipe placeholder B has => 2 FU
    pipe placeholder C has => 3 FU
    pipe placeholder D has => 4 FU
    pipe placeholder E has => 5 FU
    pipe placeholder F has => 6 FU


    In that case, the pipe sizing will be correct:


    pipe placeholder A => ě15
    pipe placeholder B => ě15
    pipe placeholder C => ě15
    pipe placeholder D => ě22
    pipe placeholder E => ě22
    pipe placeholder F => ě28




    If I can't divide the main pipe placeholder in different sectors (A,B,C,D,E,F), the size for this pipe placeholder will be just ě28 without any reducers.


    Guys, do you have any suggestion about that?
    Is there any way to divide the main pipe placeholder in more than one?




    Thanks for your answer!!!!!!

  9. #9
    Forum Addict josephpeel's Avatar
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    Yeah this is the only problem with this method.. Because you have no fittings the main placeholder is just one size.
    Theres two ways to get the right sizes;
    1. Use the placeholder method so you get just one main pipe size and convert to pipes. This will make the T's and divide up the main pipe. Then make the same schedule for pipes instead of placeholders to check and adjust the pipe sizes (You will have to change the T sizes manually)
    2. Put a coupling after each connection in the placeholder model. Then when you calculate and change the placeholder sizes each section will have its own size. Then convert to pipe.

    2. Involves less manual changes than 1. but you probably want to make the schedule for pipes as well, just to check the end result and to make changes after the pipe has been modelled.

    This is why I think making pipe calculation in the fittings may eventually be a better method. If all your fittings size automatically you can select the pipe run, do Pipe Size Calculation, and select "Connector size only". Then it adjusts all the pipes to size of the connected fittings. Still working on this though... and it requires different fittings for different types of calculation...
    Last edited by josephpeel; September 18th, 2015 at 08:56 AM.

  10. #10
    Forum Addict josephpeel's Avatar
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    Or.. Dynamo!

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