The Erase Double Line (Window) command is used to erase the 2D ductwork for all of the ducts in a windowed area.
To erase the 2D graphics for ducts, go to
Ribbon: DM HVAC->Ducts-> 
Erase Double Line (Window)
Pulldown Menu: DM HVAC->2D Drafting->Erase Double Line (Window)
You will be prompted to specify the first point of the window around the 2D ductwork to erase.
First crossing point around double line to erase:
You will then be prompted to specify the second point of the window.
Second crossing point:
All of the 2D ductwork that is inside or crossed by the window will be erased.