The Erase Double Line (Branch) command is used to erase the 2D ductwork for all of the ducts in a system.
To erase the 2D graphics for ducts, go to
Ribbon: DM HVAC->Ducts-> 
Erase Double Line (Branch)
Pulldown Menu: DM HVAC->2D Drafting->Erase Double Line (Branch)
You will be prompted to select a duct in the system from which to erase the 2D ductwork.
Select duct on branch to erase:
The 2D graphics for the selected duct and any connected ducts will be erased.