The Turn Double Line and 3D Layers On command turns the double line and 3D ductwork layers on.
To turn the double line and 3D ductwork layers on, go to
Ribbon: DM HVAC->Layers-> 
Turn Double Line and 3D Layers On
Pulldown Menu: DM HVAC->Layer Management->Turn Double Line and 3D Layers On
The double line and 3D ductwork layers will be turned on and visible on the drawing.
If this command is run while in a paper space viewport, the VP Freeze setting for the layers will be changed to Thawed.