Transitions are needed in almost every duct run to change the size or shape of the ductwork. This article explains how to lay out rectangular transitions.
A duct run is often kept flat on the top to keep it against the overhead. A transition is also likely to be flat on top. If the duct is relatively small and the change is only in the height of the duct, such as a 12- x 12-inch duct changing to a 12- x 6-inch duct (Figure 1), the transition is often made in two pieces. A two-piece transition can be cut out of a sheet economically and has only two seams.