Modeling a Return Air System in Revit MEP

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This Tutorial is by Bill Knittle a Synergis Building Solutions Engineer, you can read more about Bill at the bottom of this tutorial

This tutorial is focused on understanding how air can be relieved from the Return Air system modeled in Revit MEP.

Revit MEP systems rely heavily on the connector. The connectors in a Return Air system are going to draw airflow away from the space the Air Terminals reside in and return it to the Mechanical Equipment upstream. Therefore, the connector on the Air Terminal must be set toOut to successfully pull air out of the space and the connector on the Mechanical Equipment must be set to In to pull that air in. The connectors on the duct and fittings are bi-directional because the air can be coming from either direction.

The diagram below indicates the intent of how the Return Air system needs to work. The purple arrows represent the connectors and the direction in which the air moves through the system. The connector on the Air Terminals are set to Preset, which allows the end-user to define the airflow at that terminal to be pulled Out from the space. The connector on the Mechanical Equipment is set to Calculated to compile the total airflow drawn In from the duct and terminals downstream. This means that as you add Air Terminals to the system, the airflow at the Mechanical Equipment increases. Therefore, my Louver must draw air In from the system and allow the user to Preset the airflow at the terminal.

If you select Return connector in the Air Terminal family and go to Element Properties, you will see the specific values set for the connector’s parameters. Exploring the Mechanical Equipment family yields the same results.

Below is a completed Return Air system minus the Louver. It is drawing 600 CFM back to the Air Handler. A color fill scheme has been added to display the airflow in the system more clearly.

Now, you will need to create a family which can relieve air from the system. This family will need to be hosted by a linked architectural wall. Finally, it will have to be set as part of the Return Air System. This article is not meant to go into detail about the Family Editor nor the creation of complex geometry.

To make the system relieve air, an Air Terminal family has to be created. Begin by using theGeneric model face based.rft family template. Create some reference planes to which you can add parametric dimensions to control the Width and Height in a plan view and the Depthof the Louver in an elevation view.

Next, sketch a solid extrusion using the reference planes and lock the sketch to them. Then, add a Duct Connector to the top surface of the solid geometry that you created to represent the Louver.

Select the connector and in the Element Properties dialog box, set the Flow Configurationto Preset, the Flow Direction to In, the System Type to Return Air, and the Loss Method toSpecific Loss.

Finally, associate the connector’s parameter Flow to a family instance parameter Flow with a default value of 200 CFM. Also create a family instance parameter called Connector Size to control the Height and Width of the connector.

To complete the family, go to the Settings menu and select Family Category and Parameters. In the dialog box, pick the Air Terminal category.

You are now ready to save the family to your office library and load it into the project. Place the new Louver into the exterior wall. Use the Connect Into button on the Options Bar and then pick the main duct leading back to the Air Handler. Hey, it physically connects to the duct but, wait! There is no airflow in the main duct. What happened? Everything is set up logically. You are pulling a combined 600 CFM from the Return Diffusers. Your Louver is pulling 200 CFM out of the system. This means that only 400 CFM should be getting to the Air Handler. Huh?

Enter the workaround. You will resort to using the Split tool. Watch what happens when you split the main duct on both sides of the branch fitting leading to the Louver. Revit magically repairs the problem and solves the issue.

However, the limitation of this process is that you cannot add the Louver to the Return Airsystem when editing the system. The Flow Direction value In on the Louver’s connector conflicts with the Flow Direction value Out on the Return Diffusers. This conflict confuses Revit MEP and prohibits the Louver from being logically added to the Return Air system. It will remain unassigned on the Default Return Air system, as shown in the System Browser.