Masonry Magazine February 1993 Page. 59
withstanding a gradual buildup of flue gases to 2000 degrees fahrenheit without softening or cracking. The greater dimension of any rectangular or oval shaped flue should not be more than twice the lesser dimension. In general, most wood burning stoves would require a 16" x 20" masonry unit with an 8" x 12" flue liner. Upon installation, linings should be free from chips, openings, or cracks which may affect the tightness of the flue.
All flues must be mortared with a non-water soluble refractory cement resistant to the temperatures and acids prevalent in the chimneys constructed for wood burning stoves. The juncture of the flue ring (Thimble) with the flue liner should also be mortared in order to maintain an effective seal. The lining should be set with refractory cement sufficient only to keep the lining in place and to seal the joint. A joint thickness of Vis" to Vs" is desirable.
Refractory cement joints should be wiped smooth on both inside and outside of the flue liner. The flue liners should be placed in position first and then the masonry units placed around each section of lining. The flue liner should continue up the entire height of the chimney and end 4" (minimum) above the chimney cap. (Refer to detail C).
The flue lining for residential chimneys should be separated from the chimney by at least a 1½" air space. The air space should not be filled with insulation or any other material and only enough refractory cement should be used to make a good joint and hold the liners in position. In no way should the air space be sealed with a collar of mortar at a
Caution
The use of chemicals which are supposed to clean the chimney when they are placed on a fire could produce heat intensive enough to cause stove damage and develop thermal stresses in the concrete masonry and clay flue liners which could contribute to deterioration.
References
National Fire Protection Association, NFPA-211, Chimneys, Fireplaces, Vents and Solid Fuel Burning Appliances, February 9, 1988. New York State Cooperative Extension, Livingston County and Cornell University Task Force, Gardening Gazette, Volume
joint which might collect moisture and cause freezing damage to the chimney.
Chimney Connector
Chimney connections passing through combustible construction shall be guarded at the point of passage with a minimum clearance as shown in Detail B. The top of any chimney connection should not be less than 18" below the inside ceiling and the pipe should not extend beyond the inside face of the flue. Stove pipe connections should be as short as possible to avoid cooling the combustion by-products.
Connectors should be made of non-combustible, corrosion resistant material capable of withstanding the flue gas temperatures produced by the appliances and of sufficient thickness to withstand physical damage.
Example: Single wall metal pipe connector 6" to 10" in diameter should be made of 24 gauge galvanized steel.
The effective area of a connector for a single appliance should be not less than the area of the appliance flue collar. The connector should maintain a pitch or rise of at least 4 in. to the foot from the appliance to the chimney.
Wood burning stoves should be connected to an individual flue which serves no other equipment.
Masonry Chimneys
Residential masonry chimneys must be constructed of solid concrete masonry units that have a nominal thickness of not less than four inches. The masonry should be laid with full, push-filled, head and bed mortar joints. Reinforced residential masonry chimneys may be constructed of hollow masonry units not less than six inches of nominal thickness if the cells are fully filled with mortar.
Chimney Cap and Hood
The top of the chimney should be protected by a concrete cap which is sloped to prevent water from running down next to the flue lining and to prevent standing water from creating frost or moisture problems. A sloping cap also improves draft from the flue as well as the smoke exhaust characteristics of the chimney. The joint between the flue liner and the cap should be sealed to prevent seepage of water down beside ide the flue. The cap should protect beyond the chimney wall a few inches with a drip slot in its lower edge to keep the wall of the chimney dry and clean. (Refer to detail C).
A chimney hood can be used to pro-
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MASONRY JANUARY/FEBRUARY, 1993 59