The Move Entity Below Ductwork command is used to move an entity on the drawing below the 2D ductwork. The elevation will be set to 0. When you run the HIDE command, the entity will be hidden by any ductwork drawn over it.
To move an entity below the ductwork, go to
Ribbon: DM HVAC->Drafting-> 
Move Entity Below Ductwork
Pulldown Menu: DM HVAC->Drafting Tools->Elevations->Move Entity Below Ductwork
You will be prompted to select the objects to be moved below the ductwork.
Select objects to move below ductwork:
The elevations of the selected objects will be set so they are below all of the 2D ductwork on the drawing.