Systems analysis and gbxml

[mkrausMQRRX]

Okay, I'm not sure if this is the right place to ask this but here it goes:

My company is evaluating Revit's Systems Analysis tool in Revit 2022. Currently, we are using a third party program to run load calcs for buildings and are thinking about replacing it with Revit's Systems Analysis tool. We built a simple test project in Revit that consists of a one room building. In our energy settings we have the material thermal properties set to schematic types. Our intention is to run the systems analysis and then export a gbxml to our third party program to see how close the load calcs are. However, when we export the gbxml to our third party program we are noticing that some of the constructions, specifically the exterior wall constructions, are not coming in correctly. For example, when we use the schematic type 8 in lightweight concrete block, the gbxml imports a wall with TWO layers of 8 inch light weight concrete. (picture is from the third party program after the gbxml import)

We went into our program files and opened a document called constructions.xml. It is our understanding that this file is where all the wall constructions are defined. It appears that even in the constructions.xml document there are two layers of 8 inch concrete. 

Then we tried to change our project to a different schematic type to see if we could get the gbxml import to work with a different wall. This time we tried  4 in heavy weight concrete with air space as our schematic type for exterior walls. And again our gbxml import did not work. This time it didn't even use 4 inch walls!

Again, we looked in the constructions.xml file. This time we saw that there were two layers listed as "mat-AM14"

After digging through the constructions.xml file we confirmed that mat-AM14 was the id for a 6 inch heavy weight concrete wall - not the 4 inch heavy weight that it was supposed to be.

These are just the ones we found. This of course seems to be having an affect on u-values, building weight, etc. which is affecting our load calcs. We can always manually correct our third party program to have the correct wall constructions. However, as we understand it, to run a systems analysis Revit exports a gbxml to open studio which then exports data to energy plus (where the load calcs are actually performed) then energy plus sends a detailed report back to Revit that the user can see. And given that the walls also seem to be incorrectly constructed in the constructions.xml document we think that the issues we are experiencing are with Revit itself and not our third party program. Thus, our real concern is that if the gbxml is not properly exporting to our third party program then it may not be correctly exporting to open studio and would be giving us an inaccurate loads report.

So, is our understanding correct that Revit exports a gbxml to open studio? Is there a reason that the walls would be constructed in a seemingly incorrect way in the constructions.xml file? If not, will this issue be addressed in the near future? As usual, any advice/information is much appreciated.

[HVAC-Novice]

I only have experience with Trane Trace 700 and you seem to use a different software. but IME gbxml requires a lot of manual cleanup. IIRC in Trace I didn't even get actual wall composition. but it creates a LOT of little wall elements I had to delete. it took a lot of work to find which walls were real, and which were not. T700 doesn't show the actual model, so that didn't help either. So what you experience may be a limitation of gbxml. 

The only thing that actually worked relatively well was to get window and door sizes and those walls in the correct orientation. 

We now use Trace 3D, which just recently added gbxml import feature. but i haven't tried that yet (T3D overall is very buggy to begin with)

I'm actually curious about your test results if you find Revit could be used to perform load calcs. Especially when you have an ERV etc. So if you don't mind, keep us posted.

[JustinStirling26]

Does anyone have anymore information here on this topic? @mkrausMQRRX, any updates to share?

[JustinStirling26]

@dbutts7, do you have anything good to share by any chance?

[Kevin.Lawson.PE]

Yes the constructions.xml names can be a little misleading.  Our plugin has a tool that reads the file so that you can see all of the assumptions that Revit is making.  You can also add you own constructions to the file as you see fit.

Here's a screenshot:

And here's a video of the tool in action: https://youtu.be/qrIemhYy2jM?t=825

There’s a free trial version on the homepage at https://RippleEngineeringSoftware.com/ if you’d like to have a look for yourself.

I will say that many 3rd party loads tools do not do a good job importing gbXMLs.  Trane Trace, for instance, doesn’t import thermal properties at all. Your typically better of using the built in Revit Heating and Cooling Loads tool or the or the built in Energy Analysis tool. 

[Iev60047]

Yes, Trace 3D does not import thermal properties or space loads from a gbXML, but they do allow you to set up templates for building construction and room load types, so that if your buliding is fairly regular, you can build a decent model using Revit. See my post here for the workflow I adopted. 

It's currently nicer in my opinion to have the gbXML exported to Trace 3D (or some other 3rd party tool) where the building construction types and loads can be verified, because as per the above posts, I don't think Revit development currently cares that much about making professional grade gbXML files. I mean, how come there are simple ID mis-assignments? That seems like easy bug-fixing that could be patched in a hot-fix.

In addition, working with plenum spaces in Revit creates so much hassle and bugs, that the resultant gbXML is basically unusable. I have to dumb down my models so much in order for the simple mode gbXML model to come out nearly ready (aside from the adjacent spaces/surfaces bugs that are inherent no matter what). 

IMHO I am thinking of eventually asking my supervisor to invest in the yearly license for Sketchup. I would prefer to create my models where it's much less effort to build, modify and iterate load models using tools like Trace 3D or OpenStudio. In addition, using SketchUp, you can really build much more detailed gbXML models.

My mind is open to being changed, but until Revit puts some serious legwork in their gbXML development, I am finding it hard to even consider using Revit for systems analysis. If anyone has personal positive experience that suggests otherwise, please let us know!