Draw 3D Ductwork (Window)
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To draw ductwork in 3D, go to
Ribbon: 
 Draw 3D Ductwork (Window)
Pulldown Menu:
You will be prompted to specify the first point of the window around the ducts to draw in 3D.
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:
Any existing 3D ductwork that is crossed by the window will be erased. All of the duct nodes that are inside the window will have their fittings drawn in 3D. All of the duct centerlines that are inside or crossed by the window will have their ductwork drawn in 3D.
Show background below ductwork: Sets whether entities below the ductwork are hidden.
3D circle approximation: Sets the number of sides on polygons used to approximate circles in the 3D model.
Draw hangers in 3D: Sets whether hangers are drawn.
Draw mechanical equipment in 3D: Sets whether pieces of mechanical equipment are drawn.