Masonry Magazine February 1993 Page. 57
Concrete Masonry Chimney
Construction for
Wood Burning Stoves
Recommended practices for concrete masonry chimney construction if a wood burning heat source is being installed as a supplemental or major heating source in a home.
The new airtight woodstoves now on the market are among the best. In comparison to the older, simple stove, the modern airtight stove is considerably more efficient. However, the conditions that allow for more efficient heat control with the airtight stoves are the same conditions that increase the buildup of combustion by-products inside the chimney flue.
The airtight stoves have air inlets which can be closed to slow the burning rate of the wood. When this occurs, the smoldering fire produces dense smoke, very little heat is released to the chimney and the amount of air required for an effective updraft is eliminated.
The cooler temperatures inside the chimney increase the chance of combustion by-products condensing on the chimney surface and the eventual buildup of deposits. Wood smoke contains unburned gases and vapors which condense on contact with any surface whose temperature is below their dew point (the temperature at which vapor begins to condense). This occurs when the combustion by-products, in vapor form, contact the flue liners exposed to outside temperatures. The condensate (creosote) contains acids, tars, sulfates, moisture, and other materials that may cause severe deterioration to concrete masonry.
In order to minimize the deterioration of concrete masonry units used in chimneys servicing wood burning stoves, proper construction practices are essential.
Concrete masonry is not an acid resistant material. The masonry is provided specifically for the protection of the clay flue liner and the structural stability of the chimney. In order to insure a trouble free chimney, the combustion by-products must be effectively sealed within the clay flue liner.
Foundation
The foundation for a chimney should be made of concrete and designed to support the weight of the chimney. No other part of the structure should be supported by the chimney unless the chimney and foundation are specifically designed as a structure support. The footing thickness should be at least eight