The firebox and the chimney are connected by a slot-like opening called the throat. It is at this point that a damper followed by a smoke shelf and a smoke chamber are provided to help direct the smoke and combustion gases out of the chimney to the outside, as shown in Figure 1.
A damper is a metal door used to regulate the draft in the chimney. With the aid of the damper, the throat can be completely opened or closed, preventing heat loss from the room when the fireplace is not in use and keeping insects out of the house during summer. The damper extends the full width of the fireplace opening and is installed approximately 6″ to 8″ above the lintel, which is the piece of metal that allows masonry to span the fireplace opening.
Figure 1 - Fireplace - functional items
To prevent downdrafts form the outside form interfering with updrafts carrying smoke and gases, a smoke shelf is provided. Its function is to bounce downdrafts back up the chimney. The smoke shelf runs the full length of the throat. Its depth ranges from 6″ to 12″ according to the depth of the fireplace. In masonry fireplaces the smoke shelf is built out of brick, stone or other material and made concave with mortar.
Note: A special mixture of firebrick mortar is used throughout the fireplace and chimney construction.
Note: Prefabricated fireplaces have the smoke shelf already built in.
The smoke chamber is the space above the smoke shelf that acts as a funnel for smoke and gas. It slants from the firebox walls toward the chimney, correcting the length gap between the two while squeezing gases out of the flue.
Prefabricated chimney sections are also available for installation with fireplace units.
Additional information on fireplaces
Additional information on fireplace chimney design
Additional information on drying logs for fireplaces
Additional information on marble mantels for fireplaces
Additional information on how to build a masonry fireplace