Masonry Magazine January 1984 Page. 28
Base Assembly
At the base of the baffle chamber, clean-out doors should be installed so that soot and ash deposits may be removed. The shell of the fountain-style heater is also started at the base of the baffle system. This shell should be constructed of two wythes of brick masonry separated by a nominal 1-in. (25mm) air space and tied with metal ties. The exterior wythe should contain horizontal joint reinforcement. Because these vertical baffle chambers must extend up along the sides of the firebox, the size of the base is directly dependent upon the firebox dimensions.
Extended Hearth
As the base assembly is constructed, the support for the extended hearth is usually formed so that the top of the extended hearth may be constructed at or slightly below the clean-outs. The extended hearth is usually formed by placing a reinforced concrete slab on top of the corbeled base assembly, as shown in Fig. 1. Non-combustible, or removeable, forming should be placed so that it spans from the corbeled masonry to the floor joists forming the opening for the heater. Double joists should be used around the entire perimeter of the opening with a nailer attached to the joists to support the edge of the forming material. This type of construction is discussed in more detail in Technical Notes 19A Revised. To decrease the number of concrete slabs occurring within the heater assembly, the extended hearths may be formed by using trimmer arches, as shown in Fig. 3.
The extended hearth should be at least 20 in. (500 mm) in front of the firebox opening and 16 in. (400 mm) around the remaining perimeter of the heater. The extended hearth may be eliminated on the back of the heater if it is positioned against a non-combustible wall with a minimum fire rating of 1 hr. Once the reinforced slab is installed, the extended hearth may be finished with brick pavers and at least one course of brick masonry should be placed to form the base of the fountain-style heater baffle chamber. If removeable forming is used, the concrete slab used to support the extended hearth must be designed as a cantilever.
Ash Pit
The masonry heater, because of its efficient combustion of wood, does not require an ash pit. If an ash pit is desired, it should be positioned so that the ash drop occurs in the center of the firebox width, toward the front of the firebox. This location results in the ash pit being formed by brick masonry used to construct the portion of the baffle system in the base assembly. The ash drop should extend through the firebox hearth and hearth support. If an exterior combustion air system is desired, the optional ash pit should be eliminated and replaced with the vertical air passageway which forms the exterior combustion air inlet to the firebox.
Firebox Assembly
The firebox assembly is constructed on a reinforced concrete slab, which spans from the front to the rear of the heater. The reinforced concrete slab should be at least 2½ in. (63 mm) thick and must be thick enough to satisfy the structural and reinforcement coverage requirements. The slab should bear at least 2 in. (50 mm) on the interior wythe of the shell of the brick masonry heater. The width of the slab must be limited to the outside dimensions of the firebox width so that the baffle chamber may be continuous to the base assembly.
The typical dimensions of the firebox opening are: 12 in. (300 mm) to 18 in. (450 mm) wide, by 24 in. (600 mm) in height. The firebox is usually 18 in. (450 mm) to 24 in. (600 mm) deep, and about as wide as the firebox opening. The shape of the firebox is similar to that of a conventional fireplace without flared sides. The back wall of the firebox is sloped forward to form the throat. This slope usually begins at about 12 in. (300 mm) from the firebox hearth and should form a 4-in. (100 mm) throat that is as wide as the combustion chamber.
The combustion chamber should be constructed of refractory brick units to obtain a thickness of at least 2½ in. (63 mm). A sliding damper may be installed at the throat. The throat damper is usually installed so that it may be operated from the face of the fireplace. If a throat damper is used, sufficient provisions are necessary to allow for the thermal expansion so that the damper does not bind or crack the surrounding brick masonry. Whenever possible, a throat damper should not be used.
If air is to be provided to the smoke chamber, provisions must be made in the face of the heater to form an air passageway. The air inlet is usually through specially fabricated firebox doors and placed so that the air enters the smoke chamber at the throat where the velocity of the combustion gases is the greatest. A specially designed firebox door is usually required to provide combustion air to the smoke chamber.
Smoke Chamber Assembly
The smoke chamber assembly should be constructed of refractory brick units. The base of the smoke chamber should be constructed of fire brick. The front and rear walls of the smoke chamber should be 12 in. (300 mm) to 16 in. (400 mm) high. The side walls should be 8 in. (200 mm) to 12 in. (300 mm) high to provide an opening for the combustion gases to enter the vertical baffle chambers. The top of the smoke chamber should be constructed of a 2½ in. (63 mm) to 4 in. (100 mm) thick reinforced refractory-concrete slab. The slab thickness must be adequate for structural and reinforcement coverage requirements. The rear of the slab should bear at least 2 in. (50 mm) on the wythe of brick immediately behind the fire brick wall of the smoke chamber. The front and sides should bear at least 2 in. (50 mm) on the interior wythe of the shell of the heater. The distance from the bottom of this slab to the bottom of the baffle chamber should not exceed 5 ft (1.5m), thus controlling the distance combustion gases are circulated and the height of the heater.
The smoke chamber is often constructed with an opening directly into the chimney. This opening is