The smoke chamber is the space extending from the top of the throat, ee, up to the bottom of the flue proper, tt, and between the side walls. The walls should be drawn inward 30° to the vertical after the top of the throat, ee, is passed and smoothly plastered with cement mortar not less than one-half inch thick.
Damper
A properly designed damper, as shown in [figure 34], affords a means of regulating the draft and prevents excessive loss of heat from the room when the fire is out. A damper consists of a cast-iron frame with a lid hinged so that the width of the throat opening may be varied from a closed to a wide-open position. Various patterns are on the market, some designed to support the masonry over the opening, others requiring lintel irons.
Figure 38.—This well-designed small stone fireplace was built in accordance with the principles given in this bulletin. It is a good heater and does not smoke. The jambs are wide enough to resist the thrust of the arch.
A roaring pine fire may require a full-throat opening, but slow-burning hardwood logs may need only 1 or 2 inches of opening. Regulating the opening according to the kind of fire prevents waste of heat up the chimney. Closing the damper in summer keeps flies, mosquitoes, and other insects from entering the house down the chimney.
In houses heated by furnaces or other modern systems, lack of a damper in the fireplace flue may interfere with uniform heating, particularly in very cold windy weather, whether or not there is a fire on the hearth. When air heated by the furnace is carried up the chimney there is a waste of the furnace fuel, but a damper partially open serves a slow fire of hardwood without smoking the room or wasting heated air from the main heating system.