The throat damper should be as wide as the fireplace, so the side walls
should not be drawn in until after the throat is passed. Smoke dampers
with lid hinged at the back will help the smoke shelf to turn the down
draft; if the lid is hinged in the center the downward and upward
currents are apt to conflict. The placing of the damper varies with the
type, but generally the bottom of the frame is built into the brickwork
at the level of the top of the fireplace opening, forming the throat and
supporting the masonry above it.