Masonry Magazine October 1982 Page. 23
AIR INLET VENT
TYPICAL BRICK
BAFFLE
EXTERIOR AIR
INLET
AIR OUTLET
VENT
Air-Circulating Fireplaces
By circulating room air behind the combustion chamber, through brick baffles, additional heat can be delivered to the room or other areas of the house. An example of a masonry air-circulating type fireplace, the Brick-O-Lator fireplace, shown in Fig. 5, can be very efficient. These types of fireplaces can be used as supplementary heating systems, combining the basic principles of a fireplace with those of a forced air system.
Brick has the advantage of radiating the heat stored in the brick after the fire is out. Additional heat can be distributed by continuing to circulate air through the air-circulating fireplace even after the fire has died out.
SELECTION OF MATERIALS
# General
The proper selection of quality materials is essential to the satisfactory performance of a fireplace assembly. No amount of design, detailing or construction can compensate for the improper selection of materials.
# Brick
Most building codes require that solid masonry units be used for fireplace construction. Solid brick should conform to ASTM C 216 or C 62 for facing brick and building brick, respectively. When vertical reinforcement is to be used, there is an option of using hollow brick conforming to ASTM C 652. For durability, Grade SW should be specified since the fireplace assembly is usually subjected to severe exposure. It may be more economical to use more than one grade of brick, using Grade MW or NW where the units are not exposed to the exterior or earth.
For the combustion chamber, refractory brick, conforming to ASTM C 64, are recommended. Refractories are more resistant to heat and thermal shock. Building brick or facing brick, of Grade SW, may be used instead of refractories when severe fire exposure is not expected. It is recommended that a nominal 4-in. (100 mm) thickness be specified for combustion chamber walls, regardless of which brick is used. When refractory brick are used, some codes permit the walls to be a minimum thickness of 21/2 in. (65 mm).
Salvaged brick should not be used because they may not provide the strength and durability necessary for satisfactory performance. The use of salvaged brick is discussed in Technical Notes 15.
# Mortar
It is often more convenient and economical to use only one type of mortar for the entire fireplace and chimney. Type N. portland cement-lime mortar, conforming to ASTM C 270 or BIA M1-72, is a good, all-purpose mortar for most residential fireplaces and chimneys. Chimney wind loads, if they exceed 25 psf (1.2 kPa), may require Type S portland cement-lime mortar, while masonry in contact with earth should be of Type M portland cement-lime mortar.
For the combustion chamber, it is desirable to use a fireclay mortar conforming to ASTM C 105, instead of conventional mortars. Other refractory mortars have also been used successfully.
High-lime mortars, such as Type O portland cement-lime mortar have been found to be more resistant to heat in the combustion chamber than high-portland cement mortars. The joint size also affects the performance of the mortar. If the mortar joints in the combustion chamber are limited to no more than 14 in. (6.4 mm), they are less likely to crack and deteriorate.
# Steel Lintels
Corrosion-resistant steel conforming to ASTM A 36 should be used for lintels supporting brick masonry.
# Ties and Reinforcement
Corrugated Metal Ties. Corrugated metal ties may be used to tie brick of the fireplace face walls and the exterior brickwork to wood frame backup. Ties should be corrosion-resistant, at least 22 gage, 78 in. (22.2 mm) wide, and 6 in. (150 mm) long.
Wire Ties. Wire ties are recommended for tying brick construction together. They should be at least 9 gage and corrosion-resistant. Ties should be fabricated from wire which complies with ASTM A 82 or ASTM A 185.