This picture is of the exterior of a chimney that has been enclosed by an addition. The fireplace is built on the exterior side of the chimney, with the "hood" loosely attached to the brick. But not to worry. The homeowners used silicone caulk on the seam openings. A damper door was installed near the top of the hood, with a pull chain to open and close the vent. It is quite the engineering feat and very unsafe. As you can see by the photo, it was used often.
JRV Home Inspection Services, LLC
Wallingford, Conn.courtesy of the ASHI Reporter