Build Your Own (90% Efficient) Fireplace
(Page 3 of 3)
November/December 1980
By David Gustadson
The New Mexican stove builders have assembled their creations either over beds of bricks or on poured concrete slabs. To hold the various components together, they use thin layers of Kast-Set as mortar in the joints, though they say refractory cement will work, too. (Another two weeks of air-drying will cure the seals.)
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The flue installation is very important to the stove's ability to transmit heat properly. In a standard room with seven- to eight-foot ceilings, a 16-foot length of 6" pipe — equipped with an airtight damper and arranged in a serpentine pattern — seems to work best. Too much stovepipe will result in excessive convolutions, fsand there may not be enough hot exhaust flow to allow the stove to draw properly. (Of course, as with any heat-producing appliance, the firebox and flue stack should be 36 inches away from all combustible surfaces, and a ventilated wall thimble should be used.)
TRY IT OUT
Once the assembly is completed and checked, it's ready for the initial conflagration. Fuel for the first five fires should progress from a single sheet of newspaper to a healthy handful of kindling . . . in order to complete the curing and conditioning process. Afterward, the stove can be used normally.
According to its builders, the fireplace will — when loaded with five pounds of wood and lighted — give an instant dose of heat to the room through its stovepipe. An hour and a quarter later, the outer surface of the firebox will reach nearly 200°F. And six hours after that, its masonry "hide" will still be above 100°F ... though the flue pipe will have long since cooled. Bob and Alex suggest a fast, hot fire at first . . . until the coals stage begins, usually after about 35 minutes. Then they recommend that the flue damper be shut, to allow only a slight flow. (Normal store-bought "flappers" are too "drafty" ... but you can modify such a unit by simply placing a solid sheet-metal disk over the existing plate.) This mode of operation allows the fireplace to absorb heat rather than dump it out the chimney.
When the two designers tested their stove, they came up with efficiency figures as high as 92.6%. And, even if this incredible percentage figure doesn't pan out for everyone, the "ceramic stove" could be about the closest thing to homeheating perfection available . . . especially for a couple hundred dollars and 19 hours of easy labor!
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