The Erase Double Line (Window) command is used to erase the 3D ductwork for all of the ducts in a windowed area.
To erase the 3D graphics for a duct, go to
Ribbon: DM HVAC->3D-BIM-> 
Erase 3D Ductwork (Window)
Pulldown Menu: DM HVAC->3D-BIM->Erase 3D Ductwork (Window)
You will be prompted to specify the first point of the window around the 3D ductwork to erase.
Draw triple line:>> Select ducts: First corner of window:
You will then be prompted to specify the second point of the window.
Draw triple line:>> Select ducts: Second corner of window:
All of the 3D ductwork that is inside or crossed by the window will be erased.