Category Archives: sizing

It’s so good that its bad

Continuing from my previous post (well, along the same thought lines), I have come to the conclusion that Revit MEP is so good that it can actually be bad! Let me explain.

With Revit MEP, I can design ductwork with its insulation, and while coordinating with structural and architectural, I see that the duct is banging right through a joist. It is at this point that I need to make a value determination; do I fix it or let it go through? Let’s look at both:

Fix it
I need to get the duct through the space given me from the architectural plans and the structural. Oh, we can’t forget about electrical and their lights and cable trays. If the duct is a large duct carrying lots of cfm, then I really have to worry how to get that darn duct through the maze presented to me. One choice would be to throw it back to the architect and tell them that they need to give me more space. This might mean raising the roof, or lowering the ceilings. None of which the architects (in my experience) cherish doing. ๐Ÿ™‚ We can go to the structural engineer and tell them they need to give me more room, which means they have to spend more time re-designing and evaluating. Rarely, have I seen structural engineers willing to make their steel members shorter. I mean in both the architects, and structural ย engineers minds, they are designing for certain criteria; whether it be photo metrics or the clients desired ceiling heights. And the steel is sized to make sure the building doesn’t fall down when they get that record 3 feet of wet snow on top of their roof. I get it. So, we all are pretty stubborn in our minds. ๐Ÿ™‚ And we absolutely cannot forget the biggest factor that prevails: we all have a budget and time allotted towards the project. And it is our common goal to come away from this project with a profit. If that were not so, I would no longer exist in my company because they could not afford to keep me. So, lets talk about if the duct is a small little thing and I am presented with the same issues, the solution would be easier in my mind – let it go.

Let it go
I know we all glory and revel in the wondrous achievement of BIM and Revit, and love to spin that 3D model around to see our handiwork. C’mon, admit it you DO like that! ๐Ÿ™‚ But sometimes if it s a small issue, like the smaller duct, then I feel that we leave that judgement to the contractor. After all, the endgame of our design is a 2D REPRESENTATION OF OUR INTENT. We all know that in most cases, the contractor is going to look at our plans, roll them up and put it up the way that they feel that will make their process shorter. The drawings and the engineers stamp indicate that we have a set of plans that we present and that they are there to show the contractor that this is the route we want the duct to go. The more important things for the contractor (where they WILL take a closer look), is the equipment, controls, and balancing requirements. If you are lucky to have a competent contractor that you have worked with before on project, then the project flow is almost zen-like. They know the intent. They have done this kind of work before and there will be very few RFI’s. I know, I’m speaking about the ideal situation. But let’s get serious. The bottom line is that everybody is doing their thing to make money. So if Lowbid Contractor gets the job and you have not worked with them before, then your drawings have to spell out all the p’s and q’s otherwise you will be nickel and dimed for every change. So back to Revit. Sometimes, letting it go is easier on you and your company but COULD come back to bite you later.

Pipe Blobs
So, another scenario where Revit is so good that it is bad is in pipes. I am amused at the various tutorials showing all your pipes doubled up on a rack in real sizes, real spacing. umm, have you even tried to plot that out on your 1/8″ = 1′-0″ plan? Do you also get a large black blob on your screen where all those pipes are? So, we have to go back to the ‘old school’ CAD drafting and space all the piping out so that when it is plotted one can clearly see the pipes. Yes, one could cut all kinds of sections to show the intended pipe configuration (and it is desirable to do that). But Revit is so good at what it does that it slows you down in that aspect. Think about a pump room and the all the suction and return piping in the pump room. If you designed your piping the way that it actually WANTS to look, on a plan (even a large-scale plan, it will be totally unreadable/un-understood. The bottom line in this case is that one would have to design multiple view sections of the pump room to clearly show the intent. Once again, the goal is to produce a set of 2D plans to the contractor to show them the desired layout scheme.

Home-Run, baby!
I stole that line from a co-worker. ๐Ÿ™‚ We are experimenting with an idea that we borrowed from electrical, that should meet all the demands ย of the above mentioned. Here it is: For congested areas, such as lavatories and situations where we need to run hydronic supply and return to VAV boxes, we simply – home run it. We run the main pipes to the particular rooms and stop there. We give a detail of a common layout. We keynote all the equipment. We provide a schedule for the keynoted items with the required pipe sizes. DONE! What does this do? It saves US time. We no longer have to run that 3/4″ water line all around to each fixture. We have to give all the information to the contractor to FIGURE IT OUT FOR THEMSELVES. We don’t need (or shouldn’t need) to hold their hands on the exact routing of the VAV hydronic piping.

We are trying this process out, and hope that the authors will smile upon this and that the architects won’t be tho upset that we didn’t show every tiny little pipe, fitting, valve, etc. We feel it is a win situation, and unlike the architect that must show the clocks models on the walls, blackboards, etc. We don’t need to go to that level of detail UNLESS the client wants a super-cool 3D model where they can virtually go in each room, above the ceiling, in pipe chases to see everything. If that is the case, then we will need to discuss more money! ๐Ÿ™‚

Let me know how your company deals with these situations and perhaps we can collectively help each other out. Hope this long dissertation was of a benefit.

Annotating Duct Sizes – Really!

After completing my first project in MEP 2012, I now have a little “free time” before the next deadline hits. This blog post is about duct sizing. No, not the Revit MEP automatic sizing, rather the good old fashioned duct sizing using your favorite duct wheel, or app (yes, there are MANY apps for that).

I don’t know about other firms, but our firm likes to annotate the actual duct opening; as in the size that the required CFM is needed. When you draw ducts and give it a size, it faithfully draws the duct to the size you specified.

Our little hypothetical duct has an initial requirement of 2400 cfm. Using our handy duct sizer, we confirm that a 30″ x 14″ duct will do it nicely for us. The problem lies that we want to abate some noise from this duct, so we line it with 1″ of duct liner. So now, our actual free space for air to travel is reduced: 30″ – 1″ – 1″ = 28″. And 14″ = 1″ – 1″ = 12″. Using our duct sizer, we now see that the free area is about 1925 cfm! So, we would oversize the duct to make allowance for the duct liner. (32″ x 16″).

Now to the crux of this blog. When we add duct liner, it shows it as shown below.

Now we resize the duct to make allowances for the duct liner.

What we ACTUALLY want to see is the duct free space (30″ x 14″). In order to get this, we need to dig a little deeper into Revit MEP. We will first make a copy of the family (so we don’t destroy the original) and then will modify a parameter or two:

  • Click on the duct label.
  • From the Modify | Duct Tags ribbon, select the Edit Family tool.

  • Your screen will change and you will see a large SIZE on your screen. You are now in the editor. So, the first thing we want to do is MAKE A COPY. From the R on the upper left of your screen, go to Save As > Family.

  • Call the file whatever makes sense to you. I called mine “Duct Size Free Area Tag”.
  • Click on the large SIZE in your editor and select Edit Label. The Edit Label dialog box will appear

  • Notice on the right-hand side, you see the parameter called SIZE, but also take not on the left-hand side of the dialog box in the long list of category parameters, you see a parameter called FREE SIZE. This is the one we want to use!

  • We want to replace the SIZE parameter with the FREE SIZE parameter. Click on the FREE SIZE parameter on the left-hand side and use the right arrow tool to transfer it to the right-hand side.

  • Now select the SIZE parameter on the right-hand size and transfer back over to the left-hand side using the left arrow tool. You will be left with just the FREE SIZE parameter on the right-hand side. This is want we want! Click OK.
  • So, we now have a big FREE SIZE label on our screen.
  • Click the Load into Project tool on the ribbon.


  • In the Load into Projects dialog, select the project that you want the new label to be loaded into. In this case, it is our only project. Click OK to dismiss that dialog.


  • Your duct now indicates that it is a 32″ x 16″ duct, but the annotation indicates that it is a 30″ x 14″ duct. That is what we wanted to accomplish.

As I mentioned at the beginning, this is how our firm does it. Your firm may be different. Hope this helped someone out there. If for nothing, I now have it documented should I need to recreate that family again. ๐Ÿ™‚