A cooking oven structure of conventional construction having a top vent provided thru an upper portion of the oven wall forming a cooking cavity, a bottom vent formed thru a lower portion of the wall, a chimney mounted on an exterior portion of the wall and exiting adjacent a top portion of the oven structure and communicating with the top and bottom cavity vents, and a damper mounted in the chimney and having a right angle configuration and pivotal about its apex to (a) a first position blocking gas flow both thru the top cavity vent and the chimney, (b) a second position blocking gas flow only thru the top cavity vent, and (c) a third position blocking gas flow only thru the chimney.