You can override a routing preference by changing the Connection Type and specifying different parts to use while drawing a duct run. For example, if you want to use flange connections for a specific run instead of the default slip joint connections, you can select it in the drop down menu.
When you change a default part, it remains in effect until you end the Duct Add command. The next time you add ducts, the default part in the routing preference is resumed.
To override a default fitting
- Add duct, as explained in Creating a Duct System.
- On the Properties palette, expand Advanced, and under Routing Options, click
to open the Fittings Settings dialog box. 
Fitting Settings dialog box
The Fitting Settings worksheet shows the current Connection Type, and lists the available types and parts.
- In the Fitting Settings dialog box, select the desired Connection Type in the drop down menu.
- To change the parts, select the fitting type to override.
- Choose a part from the list.
- Specify points to lay out the duct run.
The overridden part is automatically inserted into the layout.
The override is removed after you end the DUCTADD command. The next time you run DUCTADD, the default part from the routing preference is used once again.