If you are just drawing lines in plan and saving that as an object, then that is all you will have in your object - lines in plan.
You need to model the vent in 3D, select the 3D elements and save those as an object.
This will generate an automatic 2D projection for the plan view, although you may not be happy with that and may want to change it.
To get an object that is half way descent, you will really want to learn some basics of GDL editing.
This may help ...
https://helpcenter.graphisoft.com/user-guide/72802/
Barry.
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