Is there a way to get the total volume of mechanical ductwork occupying a room? Is there another way in Revit other than manual calculations to minus the volume of mechanical ducts in a room space from the total volume of the room?
At this tmie, I have not found a way to do what you are trying to do other than manually.
If you would like this functionality added to Revit, this link is setup for you to submit feature requests, or feedback, directly to our Development group.
I encourage you to use the feedback link as our Development group is always interested and would like to hear your feedback directly, since you are better suited to state the business case for a feature request.