The Erase 3D Ductwork (Branch) command is used to erase the 3D ductwork for all of the ducts in a system.
To erase the 3D graphics for ducts, go to
Ribbon: DM HVAC->3D-BIM-> 
Erase 3D Ductwork (Branch)
Pulldown Menu: DM HVAC->3D-BIM->Erase 3D Ductwork (Branch)
You will be prompted to select a duct in the system from which to erase the 3D ductwork.
Select duct on branch to erase:
The 3D graphics for the selected duct and any connected ducts will be erased.