After selecting the required pieces of HVAC in the Model Explorer, click Fill. The Autofill with Straights. (HVAC) window displays all the identified gaps but also the specified straight lengths are created automatically to replace the implied ducting.

Clicking Unfill removes all automatically created straight lengths.

All corresponding lengths of implied ducting are highlighted red in the 3D Graphical View. Clicking Unhighlight removes all the red highlighting from the 3D Graphical View.

The red highlight is also removed from the 3D graphical view, once the Highlight Implied Ductwork. (HVAC) window is cancelled.