Firstly create a ceiling plan only showing the ceiling. Secondly in floor plan underlay with the new ceiling plan you’ve created. Underlay Orientation. Your plan will show symbols correctly on a ceiling plan but actually its remains floor plan.
via Pipes Risers/Droppers – Ceiling/Floor Plan – Autodesk Discussion Groups.
In summary, if you encounter a Revit MEP model where ducts are clearly shown in 3D or section, but do not appear in plan, before you go through all of the visibility graphics settings, view range settings, and overrides, you may want to verify that the ducts are not connected to an Other Air system.
via Revit MEP Pop Quiz – Part 2: Why don’t both ducts appear in plan view? – The Revit Clinic.
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