Transition
Transitions in ductwork are inserted automatically when the size of the ductwork changes. This typically happens at branches.
If you need a transition in the middle of a straight section of duct, use the Break Duct command to insert a break in the duct, then change the size of the ductwork on one side of the break. When you redraw the ductwork, the transition between the two sizes will be inserted automatically.
If you're inserting an eccentric transition, see the Inserting Eccentric Transitions article in the knowledge base for more information.
To insert a transition fitting, go to
Ribbon: 
Transition
Pulldown Menu:
You will be prompted to specify the duct on which a transition fitting is to be inserted.
Select duct or label to insert fitting at:
A transition fitting will be inserted on the drawing, as shown below.
Angle of transitions: Sets the default angle of duct transitions.