Vent decreaser *Needed to allow for 4 x 6 5/8 venting where a fireplace uses 5 x 8 without power venting

This part goes directly on the firebox to reduce to 4 x 6 5/8.

Then run 4 x 6 5/8 from there to the termination where it will attach to the Power Vent Kit.

The Power Vent Kit would have to be 4 x 6 5/8 size. 

If your fireplace is the Boulevard 60 or 72, it uses 5 x 8 pipe, so needs the adapter  to allow for using the 4 x 6 5/8 pipe instead.