Use the Project Browser to specify rise drop symbols used for a duct system.
In the Project Browser, expand Views (all) Floor Plans and double-click a view for the duct system.
In the Project Browser, expand FamiliesDuct SystemsDuct System.
Right-click a duct system, and click Type Properties.
In the Type Properties dialog, under Type Parameters, under Rise / Drop, click the Value column, then click to open the Select Symbol dialog where you can select a rise drop symbol.