In the diffuser schedule, there are three fields in the Face Dimensions section of the dialog box that are used to specify the size of the diffuser.

• Face Width: The width of the diffuser as displayed on the drawing. It is used as the diameter for round diffusers.
• Face Depth: The depth of the diffuser as displayed on the drawing. For rectangular and slot diffusers, it is shown in plan view. For sidewall diffusers, the depth is used when the diffuser is displayed in 3D. It is not used for round diffusers.
• Nominal Size: This is a free-form text field that is displayed only on the diffuser schedule. It is not used on the drawing.

If you have a 24″x24″ ceiling grid, you typically want your diffusers to match that size. However, your design might call for a 12″x12″ diffuser. In this case, you would specify the face width and depth as 24, and enter 12″x12″ in the nominal size.