The throat of the fireplace is that portion between the top of the firebox and the smoke chamber. It should always be as wide as the fireplace is wide and from four to five inches in depth. By the use of a Majestic Fireplace Damper and Throat combined, this detail of construction is automatically and satisfactorily taken care of, for each damper is properly and scientifically designed to meet the required conditions.
The back edge of the damper should always rest on the forward edge of the smoke shelf. The joint between the damper and the bottom of the smoke shelf should be tightly sealed with mortar so that it is impossible for flames to get to the flue, except through the damper throat.
The smoke chamber is that portion of the fireplace immediately above the throat or damper. A great deal of care should be used to see that pro[x*r dimensions are maintained, that the front of the wall is not drawn in so abruptly as to interfere with the rising smoke, and that all surfaces are smooth and free from large projections.
The bottom of the smoke chamber is called the smoke shelf that is, the portion of the smoke chamber between the damper or throat and the back wall of the smoke chamber. It is absolutely essential to provide a smoke shelf, because in every flue there is what is known as a back draft. Back draft is caused because the front wall of the flue is warmer than the back wall when the fire is burning, and the gases Missing upward along the front wall create a suction down the back wall, resulting in a down draft of outside air. When this down draft meets the smoke shelf, it is deflected by the damper valve plate and carried up again with the rising gases from the fire.
The smoke shelf should be the full width of the throat, never less than eight inches deep, and it may vary from this dimension to 12 inches or more, depending upon the depth of the fireplace itself. See illustration.
