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Thread: Heating and Cooling Loads

  1. #11
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    Quote Originally Posted by RobDraw View Post
    I wasn't referring to only today. If you look at the number of views of recent posts, you can see what I'm talking about.
    I agree. This forum has a small number of MEP contributors. I try to here and there but I spend most of my time on Reddit. /r/revit is somewhat active. /r/revitMEP is pretty much a ghost town though.

    To your original point, Spaces and gbXML exports are very much a part of my workflow. First I get the spaces set up in the model, using Space Separator lines to carve up the large spaces into perimeter and interior spaces. Then I enter occupancy, lighting watts/sf, equipment wattage into the relevant Space parameters. I'll also define time schedules in the Space Type Settings. I like to use a space schedule to enter the data and verify that nothing was missed. I'll then make a Space Plan that uses Space Tags that list these load parameters. Then I take that plan and the schedule and sit down with my supervisor and get their approval on the load data and zoning. The point here is to make them put their eyeballs on a plan and schedule that lists all of that data, so they can't come back to my desk 4 months later and ask me "What load did you use for your calcs in Room xxxx?", or "why did you zone it like this?", which happens all of time.

    Once that's all been set, THEN I pull the trigger and export the Space data to Carrier HAP or Trane Trace. The only data that I can reliably get to translate over are Room Name/Number, Area, and the above mentioned time schedule, occupancy and wattage data. Anything to do with the building envelope is junk. Interior Spaces will have "exterior" walls, exterior Spaces will have 8 walls all facing North with an area of 6sf. Spaces with a slab above them will have multiple small "roofs". It's all junk and I have to go Space by Space and enter the envelope data manually, but this isn't so bad when you consider how much data entry time I saved simply by having the gbXML import take care of the Space creation and load data. And really, the envelope information is so critical to the design that I'd have to be double-checking it anyway.

    Once that's done and my System is set up in the load calc software, I run the loads. I then take the air flow rates that the load calc software outputs for each space and enter those back into the Revit Spaces, in the "Specified Supply Air Flow" parameter. This can be done pretty quickly using a schedule. Now I can schedule both that "specified" air flow rate and the "Actual Supply Air Flow Rate" which comes from the modeled Air Terminals in the Space. This allows me to compare the two values in real time as I add Air Terminals and balance the air flow between them.

    So that's my workflow. I'm not sure if anyone else is doing it this way but that's how I do it. I don't mess with the plenum spaces above the ceiling, I think that would add a crazy level of complexity when the whole point here is to streamline and automate the process. I also haven't tried to do an energy model in Revit, nor would I try. I haven't messed with Revit's native load calc function, nor am I terribly interested in it. There's no where near the level of functionality that we need, and there's no way that my supervisors would go for it. IMO Autodesk needs to license a 3rd party software that engineers know and trust (like Energy Pro, Trane Trace, HAP, etc) and integrate it into Revit.

    Hope that helps!
    Last edited by Necro99; January 2nd, 2019 at 07:40 PM.
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  2. #12
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    Quote Originally Posted by Necro99
    ...Anything to do with the building envelope is junk. Interior Spaces will have "exterior" walls, exterior Spaces will have 8 walls all facing North with an area of 6sf. Spaces with a slab above them will have multiple small "roofs". It's all junk...
    It's possible this is your experience because you aren't using the architectural model for space boundaries, relying on your own space separators instead? The interior/exterior interpretation should be better than junk but that does depend heavily on how well the linked model is done...and that can vary wildly.

  3. #13
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    No, the architectural Link is being used for Room/Space bounding. The separators that I place are purely for carving up large spaces internally. To be honest I haven't looked that deeply into it because it would probably involve getting our client architects to change the way they're modeling their walls and windows, and usually they're very unreceptive to any Revit-related requests that we MEP people make. Very often they don't really know how this program works, it all just works "out of the box" for them. And some of them can be very sloppy about modelling.

    What I ended up doing was creating a "line based" Detail Item family that works very much like an AutoCAD dimension. You enter the height of the thing being measured (it has Types for wall, window, spandrel, etc) and stretch it into position. It will calculate the area and then a Detail Item Tag can display it in a plan view. I just copy those around the perimeter of the building in a special Space Plan view and then refer to that as I hand-enter the envelope data. It sounds like it takes a long time but it really doesn't. Only about 20% of the spaces have an exterior exposure. Plus that area data is now in the model ("single source of truth!") and not floating out in the network somewhere in some "Area Takeoff" DWG file. Again, it's nice to have that information readily available when someone walks up to my desk 4 months after the fact trying to accuse me of screwing up my cooling loads.
    Last edited by Necro99; January 2nd, 2019 at 08:15 PM.
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  4. #14
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    It would be worth experimenting a little with a very simple model (you make yourself) and link that to see if you can better than junk results? I'd be curious if you can define a strategy that's a bit more consistent for you in the long run.

  5. #15
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    Thank you Necro!!! This is exactly what I was looking for. I'm going to be digging into your responses tomorrow. I have a model that I think is ready for export to gbMXL. I followed a different workflow that is outlined in the AKN. I'll post after I find out how successful I was.

  6. #16
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    Necro (and/or RobDraw) - Thanks for the insight. Very interested in how best to leverage the model for HVAC loads. Few questions...

    • How do spaces react to large changes in the linked arched model? Is it logical or a mess?
    • Is it much trouble to chase floor plan changes as you go or do you generally wait until the floor plan settles down before you dive in with the spaces?
    • How much computational overhead do spaces put on the model? Noticeable? How about the difference between a 20,000 sq.ft. and 500,000 sq.ft. buildings?


    Thanks

  7. #17
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    • How do spaces react to large changes in the linked arched model? Is it logical or a mess?


    I've been fortunate, for the 6 or 7 jobs that I've done this on there haven't been any major changes post-export. I have seen this happen to a coworker and it did cause him some heartburn because he was going back and forth between his Revit model and his load calc software trying to manually update the load data. For major changes, I think the best solution is to simply erase all spaces from the load calc program and re-import them from Revit. Since all of the internal heat gain data comes over fine (occupancy, time schedules, lighting/equip wattage) all you really have to worry about re-entering is the envelope data. If you document those wall/spandrel/window areas in an "area takeoff" view like I described above then re-entering that data doesn't take that long. Plus, since you're doing a wholesale replacement of the spaces, you're less likely to miss something.



    • Is it much trouble to chase floor plan changes as you go or do you generally wait until the floor plan settles down before you dive in with the spaces?


    Creating spaces and making HVAC zone assignments is one of the first things I do. Create space schedules grouped by HVAC zone and name the HVAC zones after your future VAV designations. Get this validated by your Engineer to prevent nasty "why did you do it that way?" surprises during Quality Control review. I like to wait as long as possible before I pull the trigger and do my gbXML export. Of course it's always possible that your architect will make a change but that was true in the CAD days as well.



    • How much computational overhead do spaces put on the model? Noticeable? How about the difference between a 20,000 sq.ft. and 500,000 sq.ft. buildings?


    None that I can tell. I'm running a fairly decent workstation with 32GB of RAM but even with previous machines I never saw a problem.
    Last edited by Necro99; January 4th, 2019 at 07:05 PM.
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  8. #18
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    Quote Originally Posted by Steve_Stafford View Post
    It would be worth experimenting a little with a very simple model (you make yourself) and link that to see if you can better than junk results? I'd be curious if you can define a strategy that's a bit more consistent for you in the long run.
    I'd love to find a way forward with this but, I haven't really dug into it. The approach I've developed so far is already such a massive improvement that I'm a happy guy as it is.

    But, now that I think about it... Since the gbXML file uses XML formatting, it can be opened by Excel. Dynamo can also write to Excel documents, so it may be possible to use Dynamo to handle the parts of the data transfer that Revit is failing at for some reason. I'd create a series of Project Parameters for Spaces for wall and window areas across the 8 cardinal directions. So say 16 or 24 parameters. In Revit, I'd hand-enter the areas into those parameters. Then I'd do the gbXML export. Then I'd use a Dynamo Script to take my home-made parameter areas and paste the data into the gbXML file, overwriting the "junk" data.
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  9. #19
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    Quote Originally Posted by Necro99
    ../The approach I've developed so far is already such a massive improvement that I'm a happy guy as it is...
    Easy to get comfortable eh? Good luck with your new thoughts.

  10. #20
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    Quote Originally Posted by Steve_Stafford View Post
    Easy to get comfortable eh? Good luck with your new thoughts.
    Well I have to leave something for me to do myself. It's job security, man!

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