Outdoor Wood-Burning Furnaces: Beating the High Cost of Energy

Learn the mechanics and benefits of an outdoor wood-burning furnace.

| January/February 1977

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    Marcus Ashley holds the heater's main access door open to reveal the smaller combustion chamber door and an abundance of insulation.
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    These schematic diagrams show the relative positions of the furnace's working part, and the manner in which boiler-warmed crater is routed between-furnace, house, and hot realer healer.

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Marcus Ashley's solution to the skyrocketing cost of home heating is the “overgrown doghouse with a smokestack” that sits in his backyard. That doghouse is really a scrap outdoor wood-burning furnace that provides Marcus and his wife with all the hot water they can use. It keeps their Massachusetts home at a constant and comfortable 68° F, and warms a greenhouse that's attached to the residence.

“Everybody told me this setup wouldn't work the way I thought it would,“ grins Marcus, “and maybe they were right. After all, on really cold days I do have to go out once in a while and throw in an extra stick of wood. But I guess it works well enough.”

"Well enough" is an understatement. Ashley's homemade furnace is actually an extremely efficient "self-regulating, controlled combustion chamber" marvel that incorporates a number of ingenious features.

How a Wood-Burner Heater Works

The heart of the wood-burner is a heavily insulated industrial oven salvaged from a capacitor factory. The water boiler is also salvaged, which is positioned directly over this firebox, and also heavily insulated. As a matter of fact, the whole outdoor furnace is enclosed so efficiently that, even in the dead of winter when the furnace is fully stoked, not enough heat escapes from the "overgrown doghouse" to melt the snow on the little building's roof!

Ah, but Marcus Ashley's wood-burner has other secrets too. The furnace's water boiler, for instance, contains a mechanical thermostat which controls the amount of air that can flow into the firebox below. As the temperature within the boiler drops toward 160° F, the air inlet on the firebox is opened, which increases the rate of combustion in the old oven and delivers more heat to the water circulating through the boiler. And, as the temperature in the water-filled tank rises toward 160° F, the air inlet is progressively closed, which slows the burning process and thereby limits the heat which reaches the boiler.

Actually, there's more to the regulation of the combustion that takes place in Marcus's furnace than even the above paragraph indicates. Because the flow of air through the unit's firebox is so precisely controlled, that under normal conditions, the wood within never really bursts into flame. It only smolders, slowly turns to charcoal, and is almost completely converted into usable Btu's.

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