To create a takeoff area, go to
Ribbon: DM HVAC->Takeoffs-> 
Insert Takeoff Area
Pulldown Menu: DM HVAC->Takeoffs->Insert Takeoff Area
Insert Takeoff Area Dialog Box
See the Common Takeoff Information section for more information about how this dialog box works.
Enter a name for the takeoff area and press the OK button.
You will be prompted to specify the first point of the takeoff area.
First point of takeoff area:
You will then be prompted to specify the next point.
Specify next point:
Continue inserting points until the takeoff area outline is defined. Press ENTER to complete the takeoff area outline.
A polyline will appear representing your takeoff area. The polyline is on a non-plotting layer and will not display on printed drawing files.
When a takeoff is performed in the area, all of the ducts, diffusers, and fittings in the area will be included.