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Duct Sizing Made Easy in Revit

Sizing ducts can be time consuming. Do you try and size your mains before you model them? Do you size them after the whole design is done? What if you happen to miss a branch? Maybe your calculations are off? I cannot ever seem to get my ductulator to slide and stop where I need it to without fiddling with it for 2-3 mins. There’s an easier way! Use Revit to quickly size your ducts.

Now I know there is some hesitation to letting a software do calculations. You may have questions like: how can I be sure they are correct? What parameters are they using? Etc. These are reasonable questions, but I believe using Revit to size ducts can accelerate workflows and get projects out the door a little bit faster. Let me show you how.

Duct Sizing in Revit

Revit has a duct sizing tool built in under the Modify|Ducts tab, labeled as Duct/Pipe Sizing. You can select it when you have a duct selected in the model. Before we click this, there are a few things we need to make sure are done:

  1. Make sure the system you are sizing is fully connected and that there are no open ducts, other than the branch leading to the one you are trying to size.
  2. Assign the CFM to each supply diffuser

Revit calculates the size of the ducts from diffuser back to the start of the system. That is why assigning the CFM to each diffuser is important. After this is done, select the entire run for the system that you are sizing. Then, open the sizing tool and look at the different options to set for your parameters.

Above we can see what this will look like. There are different options inside the tool. I typically size based off friction only and set branch sizing to calculated size only. If you have space limitations, you can set a height or width restraint. In the example above, I sized the flex duct to match the neck size, set the CFM to 200 for all the diffusers and did a random size for the for the branch connecting the diffusers and the branch that is supplying the branch. I set my parameters to friction only and set that at 0.08 in-wg/100ft.

After clicking OK, Revit sized my ducts to provide the right airflow for this section. Revit left the diffuser connecting flex duct at 8” diameter, as this is the correct size. It did change the next duct to a 14” x 14” and the supply duct to 14” x 14”. We can easily check this to make sure Revit did its job correctly. Using my ductulator on my desk (feel free to check this for yourself) I come up with a 14” x 14” duct to supply 1200 CFM.

Zachary Barker

MEP Technical Specialist

Certifications

Revit MEP Mechnical Professional Certification

About Zach

Zach has a Bachelor of Science in Mechanical Engineering from Arkansas Tech University and over three years of experience in the AEC industry. He has experience in the Civil, Architectural and MEP disciplines and maintains current knowledge of Revit, AutoCAD, Navisworks, Recap and BIM 360. 

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