Revit MEP

Revit MEP

Wednesday, December 16, 2009

Don't Go Half Way Using Revit MEP - Creating Systems is a Must

I just got done verifying an process that I've heard mixed issues that result if your workflow process is not correct when using Revit MEP. And it's great news in my opinion.

When you place plumbing fixtures in your Revit Model, it is placed in the Default System.

You can connect multiple plumbing fixtures to each other with pipe, and get all the wonderful flow information - all without adding those fixtures to a specific system that you create. If you select the pipe and look at it's Instant Properties, you can not only see the flow going through those pipes, but you can see that it has a "System TYPE" of Domestic Cold Water and a "System NAME" of Default Domestic Cold Water. Make sure not to confuse the two.

The default View Filter displays System Types of Domestic Cold Water in a blue color. So everything looks good, but there is a limitation of how many plumbing fixtures you can connect to each other that are put on the Default System Name. And that limitation is 50 fixtures.

Below is a thread from Jason Martin at Autodesk explaining the history of this Warning further:

One of the things that is most expensive in the “calculation process” is maintaining the “pressure losses” in sections of a duct or pipe system. To do yhis we divide any connected system into sections at any junction or size change in the system (there are additional reasons for a section to be created, like a resistance change, and I’m sure a few others). For each of the sections we track things like pressure loss, static pressure etc. Maintaining these types of calculations for relatively small sets of elements is relatively inexpensive compared to the time it takes to do things like actually move the things. In systems with thousands of elements, however, these calculations became exponentially more expensive as the element counts increase. We decided at that point that when there are more than a set number of “system assignable-1” elements assigned to a system that we would turn off these types of calculations. After quite a few rounds of testing (as well as some arguing) we decided that a “tolerable” number of elements to turn off these “complex” calculations at was 50. So, now, as soon as the number of elements in a system goes above 50 you’ll get an error message that says something like:

“The default system "Default Supply Air" is now over 50 elements. To improve performance, Revit is no longer calculating the critical path pressure drop and the more complex duct sizing has been disabled. If you want to use these features, you must define logical systems in the model instead of using the default system.”

If you receive this error message it doesn’t have anything to do with things not updating in elevations / sections, and it doesn’t make the application automatically disconnect things. In versions prior to 2009 WU3 (or whichever one it was) it was very true that creating systems improved performance “massively”. Since the introduction of the “50 element” rule, it isn’t as significant as it once was.

For other “performance” things, it really, really, really depends. In 2010 a significant portion of effort was spent on improving the performance of what we referred to as “model manipulation”. This includes things like dragging, connecting, moving, sizing (either with sizing tools or just changing the size), and deleting items like pipe, duct and fittings. As a few others have mentioned here, “model manipulation” in 2010 is significantly faster than it was in previous releases.

If you are moving a connected duct / pipe / fitting - our internal testing has shown that it is actually “slightly” faster to not have systems defined than it is to have systems defined. If you are making new connections between things - our internal testing has shown that it is actually “slightly” faster to have systems defined than it is to not have them defined. If you are changing the flow of elements in the system (like air terminals) - it is “slightly” faster to have systems defined than it is to not have them defined. If you are creating systems - it is significantly faster to create them before things are connected with ducts and pipes.

System assignable elements are things like air terminals, mechanical equipment etc. Things that when you select them you see a create system button (or edit system button) but don’t include things like ducts, fittings, pipes, etc.

Once you create your user defined system, you can move your plumbing fixtures to that system and they are moved out of the default system.

Your piping properties will now take on that System Name as well.

With this, you can create piping annotation tags that can be smart and extract the pipe size and the pipe system. Once I connect a tag to that pipe, it will read, 1" CW. And the tag will automatically update and change if I ever make a change to my design. I don't need to worry about the annotation being incorrect. I like to use these piping annotation tags in my "plot" views for my construction documents.
I also use another piping annotation tag that displays fixture units and flow at certain points of my system in my "modeling" views. That way I can make sure my system is connected, and I'm engineering it properly.
I asked Martin Schmid from Autodesk if you could theoretically put every cold water plumbing fixture in a job in one user defined cold water system, and not have a large performance hit. He says there is a trade-off - and that, even though you can put all of your mechanical fixtures in a user defined system name, the more connections and fittings you have, the longer the calculation time for edits done to the system or its layout. But it's not as bad as when you are using the default system name. Martin tells me that 2010 is faster than 2009 in this process. And you won't notice hardly in smaller jobs.
Another workflow process is that you could break up your systems into CW1 and CW2 and CW3, etc... one for each riser in a particular area. But who wants to have a system name CW4? So you will need to decide if you want to break up your systems, or depending on your project size, computer speed, OS and patience, put all of your fixtures in one user defined system.

Which brings me to the name of the article, Don't Go Half Way. I hear a lot of firms that just want to model certain parts of the building in Revit, and do 2D for the other in AutoCAD. Or they want to just create a 3D model, and not use the flow calculation tools that Revit has. But there are a lot of things going on in the program that if you're not aware of, you run into these limitations such at connecting 50 elements to a default system and you end up creating more headaches than you thought you were going to avoid by taking shortcuts. So go the extra step and create the systems and use them to your advantage. You can save so much more time accepting the full Revit process the way it is supposed to be used, than trying to come up with workarounds and unproven processes of how Revit is not supposed to be used.


  1. Glen W - Interface Eng.December 17, 2009

    Great Post, thanks for the insight on the system creation. There has been so many "best practices" for creating systems and performance issues. It seem it changes with every new release of Revit MEP. I guess for the good.

  2. great point. but how to assign a piece ductwork to a system if there is no grilles nor equipment attached to it, like a transfer duct.

  3. I agree with Dawen. We too are scratching our heads about how to treat transfer ducts. We also have a problem of accounting for flow diversity in duct and pipe systems. In this age of variable flow, it is surprising that we cannot vary the flow in mains so as not dramatically oversize duct and pipe mains and risers.

  4. Nice Post.Thanks for Sharing detailed information on system creation.

  5. The problem with what you are suggesting is that it takes additional time, and in the world of MEP design, time isn't always there. Quite simply, Revit creates problems where none existed when using Autocad.

    With Autocad you didn't have headaches from:
    No family issues
    Moving fixtures/piping when an architect/owner shifts items (ok still a headache, but not to the scale revit gives us)
    No system type issues, this is a huge headache when someone accidentally connects one system to a different one.

    Those are just the tip of the iceberg, not to mention the beastly computers and still slow-performing program. Plumbing drawings, at least in certain areas of the industry are diagrammatic. With Revit, you cannot print a 2D drawing with a rack of 3/4" piping and have it be legible if you set the piping distances to what they'll be in the real world, so you have to purposely space piping further apart to get the drawings to look correct. At that point, why use revit? If you have to do a combination of diagrammatic and realistic pipe routing, and it takes more time to draw these in revit (assuming we follow your suggestions about systems), then I have to ask, what practical benefit is there?

    As a coordination tool, Revit is great for pipe routing, you can review the model with sections (the best feature of revit by a long shot), as you draw in CAD. You can verify you have the space, identify locations where offsets need to be shown, and achieve (at least in my opinion), a better diagrammatic set of plumbing drawings in far less time.

    This is just my opinion, to each their own. If you are doing very detailed plumbing work (labs, boiler rooms, etc) I can start to see benefits. But for your average plumbing layout, it simply isn't there.