Vent on outside of chimney?

Good morning,
We are picking up and getting ready to install, (DIY), our HI-300 insert. My question is that we have a louvered vent on the exterior of our masonry chimney. I am assuming that it has to do with outside air and the drafting of our regular fireplace. The bottom of our firebox has the sliding cast iron vent / cleanout that I have used to regulate the fire with. Should we be doing anything with this and the outside vent before installing the insert? We will be installing a block off plate per recommendations on this site.