Masonry Magazine October 1982 Page. 20
COMBUSTION CHAMBER
INNER HEARTH-ASH DUMP
AIR INLET DAMPER
HIGH FORMED DAMPER
SMOKE SHELF
TABLE 2
Suggested Width of Fireplace Openings
Appropriate to Size of a Room.
THROAT
Size of Room
in Feet
Width of Fireplace Opening in Inches
in Short Wall
in Long Wall
10 x 14
24
24 to 32
12 x 16
28 to 36
32 to 36
12 x 20
32 to 36
36 to 40
12 x 24
32 to 36
36 to 48
14 x 28
32 to 40
40 to 48
16 x 30
36 to 40
48 to 60
20 x 36
40 to 48
48 to 72
EXTENDED HEARTH
"Reprinted by permission of Structures Publishing Company
from, Book of Successful Fireplaces, 20th Edition.
*SI conversions: mft. x 0.3048; mm in. x 25.4
Typical Firebox Assembly
FIG. 3
Firebox Assembly
The firebox assembly consists of the fireplace opening, fireplace hearth, combustion chamber, throat and smoke shelf, as shown in Fig. 3.
Fireplace Opening
The fireplace opening is a very important factor in the design of a fireplace. The design of most of the other components is based on the size and shape of the opening selected. Figure 1 shows details and Table 1 provides the widths and corresponding heights of fireplace openings found to be the most satisfactory for appearance and successful operation. These dimensions may be varied slightly to allow for regular brick coursing.
Proper Sizing
Careful consideration should be given to the size of the fireplace best suited to the room in which it is to be located. Location and size are important not only from the standpoint of appearance, but also of operation. If the fireplace opening is too small, it may function properly, but will not produce enough heat to warm the room. If the opening is too large, a fire that would fill the combustion chamber might be too hot for the room. Moreover, it would require a larger flue area and consume large amounts of interior air unless exterior combustion air is provided. Table 2 gives suggested widths of fireplace openings appropriate to various room sizes. For example, a room with 300 sq ft (28 m²) of floor area is well served by a fireplace with an opening 30 in. (750 mm) to 36 in. (900 mm) wide.
Support Above Fireplace Opening
The brickwork above the fireplace opening must be adequately supported. There are several alternatives for support. These include brick arches, reinforced brick masonry lintels and steel angle lintels.
Brick arches usually require no steel reinforcement and can be an attractive option. When determining the height of the opening, it is necessary to use the maximum height of the arch soffit. Information on arch design may be found in Technical Notes 31, 31A and 31C.
Reinforced brick masonry lintels may be built in place or precast. The advantages of using reinforced brick masonry lintels are numerous, but include more efficient use of materials and a brick soffit without an exposed steel angle. Reinforced brick masonry lintel design information is found in Technical Notes 17H. When reinforced brick lintels are used over the fireplace opening, the thermal stresses and subsequent differential movement should be considered.
Steel angle lintels are the most prevalent means of support. For this reason, Table 1 gives recommended steel angle dimensions for successful fireplaces. If opening sizes other than those listed in Table 1 are used, steel lintel design information may be found in Technical Notes 31B. General recommendations are: the steel angle lintels should be at least 4 in. (6.4 mm) thick, with a horizontal leg of at least 31½ in. (90 mm) for use with nominal 4-in. (100 mm) thick brick face wall and 2/2 in. (65 mm) for use with nominal 3-in. (75 mm) thick brick face wall. The minimum required bearing length is 4 in. (100 mm). Maximum clear spans of steel lintels may be restricted by the fire protection requirements in some building codes.
Fireplace Hearth
The hearth consists of two parts, the inner hearth and the extended hearth. The hearth may be raised or flush with the floor. A fireplace hearth flush with the floor is shown in Figs. 1 and 2.
Inner Hearth
The inner hearth is within the fire area and is the floor of the combustion chamber. Most building codes require that the inner hearth be noncombustible and a minimum of 4 in. (100 mm) thick. This thickness includes the inner hearth and the noncombustible hearth support.