I've taken a stab at trying to create a parametric duct fitting family pictured below with limited success:
Split Gang Fitting.png
Duct 1 Width and Duct 1 Height would be manually entered based on what is required for the airflow.
Duct 2 Height and Duct 2 Height would be equal to Duct 1 Height.
Duct 2 Width would be manually enter, while Duct 3 Width would be equal to Duct 1 Width minus Duct 2 Width.
I will preface this with the fact the I started with the "Rectangular Elbow - Mitered - Single Thickness Vanes.rfa" from the Imperial library in Revit MEP 2012. I went through and added the extrusions, parameters and constraints in order to accomplish the above mentioned results. The fitting worked for the default dimensions that I entered in the family (Duct 1 = 24x12, Duct 2 = 12x12, Duct 3 = 12x12); however, when I load it into a project and modify the Duct 1 and Duct 2 dimensions, I get the results shown below:
Split Gang Fitting-Result.png
Any insight or suggestions as to what I'm doing wrong?
Attached is the .rfa for anyone who wants to look at it.