In Revit 2009, VAV box offset means distance between centerline of it in height and floor.
If you input same offset for duct, everything is ok and no additional fitting is needed.
But in Revit 2013, VAV box offset means distance between bottom of it and floor that makes an
additional fitting between VAV box and duct.
How can I change offset position to make it work like Revit 2009.
That would be based on the Family itself, not a setting in the program or project. If you open the Family for editing and go to the Front View (Elevation), the model should be centered vertically on a Reference Plane which is marked with "Defines Origin" and "Is Reference: Center (Elevation)" under the Reference Plane's Properties. See image of the RMEP 2013 VAV Box Family.
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