Wood-Burning Furnace
(Page 2 of 3)
January/February 1979
By the Mother Earth News editors
"The design of the furnace is all-important," says Allan, "not only to promote a high efficiency of combustion, but to be able to handle that combustion safely. I didn't want any exposed steel in the interior, of course, because the temperatures inside the combustion chambers approach 2500F ... and steel melts at about 2550F. So instead, I faced the steel plate in the furnace with some chimney flue tile, refractory brick, and industrial furnace cement. Then I insulated the fire box from the furnace's sheet-metal cover with 1-1/2 inches of rigid asbestos. The whole thing is supported on an angle-iron frame, which is separated from direct heat by the refractory.
" Here's how Brown's "state of the art" wood-burning system works: When Al lays a blaze in the firebox (he calls it the "primary combustion chamber"), fresh air is forced through the holes in the flue tile by a 12-volt, 100-cubic-feet-per-minute blower fan which is, in turn, connected to a thermostatically controlled damper. (This valve stays fully open when the water temperature in the iron boilers is 140F or less, and fully closed-with the blower motor shut off-when the water reaches 190. This same thermostat control will switch the conventional oil-burnIng furnace on if the temperature of the water ever drops below 140.)
The heat from the primary combustion chamber then movesby convection-up into the secondary combustion chamber above the firebox. There, additional oxidation takes place ... since that chamber also receives air through a one-inch pipe that (because its surface is perforated with quarter-inch drilled holes) serves as a manifold. At this point the flue gases are almost completely combusted ... and they can pass into the radiator section, heat the water, and exit through the chimney, leaving a minimum of creosote or residue behind. Allan figures that temperatures reach up to 325F in the exhaust stack, and he's calculated CO 2 readings in the flue gases at 18.5% ... which means that the system operates at about an 80% combustion efficiency!
But the furnace is just the beginning ... Al has ingeniously rigged the wood burner up to his existing hot water system like this: The heated water is pumped from the radiators in the furnace to a well insulated, 275-gallon storage tank in the basement (there's also a 55gallon "head tank" plumbed into the line between these two points ... to serve as a reservoir for the radiators in the event that the circulating pump fails). From the storage tank, heated water is pumped to the three zones in the house, then returned to the storage tank and pumped back into the furnace to be reheated. In order to keep the water in the system at the correct level, a "makeup valve"-which feeds from the cold water supply-is installed to supplement the water returning to the storage tank.
In addition, Allan has built in another energy-saving feature: Instead of taking his domestic hot water from the "hot" side of the furnace plumbing (and having to wait for the heated water to travel a distance), he has installed an "on demand" heating coil in his hot water feed lines that only heats the water being used at the time (this water Is also taken from the regular cold water supply).
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