Wood-Burning Furnace

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The forced-draft fan provides air to the firebox for near-complete combustion.  
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Allan shows us how he plumbed his heavily insulated hot water storage tank to the wood-burning furnace.
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Circulating pumps send water to the three zones of the house.
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The "heart" of the system is the furnace itself, which has an incredible 80% efficiency.

“Raising our family in a huge 130-year-old farmhouse in northern New England is great,” says Allan M. Brown of Lincoln Center, Maine, “but the fuel bills were driving us straight to the poorhouse.” So, Allan went to his drawing board, and—after combining a little ingenuity with a lot of hard work—he came up with a wood-burning furnace that not only heats well, but does so at minimal expense! 

Of course, being an engineer by profession—and living on 114 partially wooded acres—made Allan’s “victory” over the utility companies all the sweeter. But, even if you don’t have Al’s “advantages,” this sophisticated wood burner is well worth looking into.

Now, Mr. Brown is quick to point out that his design is a furnace, not just a stove, and is capable of heating an entire house on even the bitterest of winter days. Furthermore, he has eliminated all the problems that normally go with wood heat: smoke and dust in the house, uneven distribution of thermal energy, and the old bugaboo of lugging great armloads of logs into the house at every turn.

Before he ever picked up a tool, though, Allan sat down and listed exactly what he wanted to accomplish. When the furnace was completed, he had succeeded on all counts:

[1] The house’s existing hot water baseboard system distributes the heat throughout the building.
[2] The existing oil burner kicks in automatically if the wood fire should happen to go out.
[3] The wood furnace is located on the ground floor outside of the main house to assure convenience and cleanliness.
[4] A high combustion efficiency is achieved, which reduces wood consumption and minimizes creosote buildup in the chimney.
[5] The firebox is large enough to make frequent fillings unnecessary.
[6] The entire system (with the exception of loading the wood) is fully automatic.
[7] The unit is completely safe.

The “heart” of Allan’s system is, of course, the furnace, which heats with a vengeance because it [1] utilizes a large (22 1/2″ X 27″ X 32″) firebox which is [2] part of a forced-air-fed primary combustion chamber which [3] feeds into a normally ventilated secondary combustion chamber, and on into [4] a two-part heating chamber that uses two recycled steam radiators as heat exchangers.

“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 2500°F and steel melts at about 2550°F. 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 140°F or less, and fully closed—with the blower motor shut off—when the water reaches 190°F. This same thermostat control will switch the conventional oil-burning furnace on if the temperature of the water ever drops below 140°F.) For a clear picture view this Wood Burning Furnace Diagram.

The heat from the primary combustion chamber then moves by 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. They 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 325°F in the exhaust stack. He’s calculated CO2 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). Here is a Flow Diagram of the whole system.

And that’s about all there is to the mechanics of the setup. The entire system (except for feeding the firebox with wood) is fully automatic and controlled by a thermistor which is connected—through an electronic circuit—to the conventional oil burner and the fan-and-damper system on the primary combustion chamber. This forced-draft fan (and its damper control) is powered by a 12-volt automobile battery that’s recharged at timed intervals by a trickle charger; the all-important blower system isn’t dependent on public utility power.

Since Allan built his wood burner from scratch, he was actually working on a trial-and-error basis. But, as it turned out, everything was so well planned on paper that there was little room for error. In fact, the only real problem that the engineer encountered was in his primary air feed tube. This was originally a section of iron pipe. It disintegrated because of the intense heat in the firebox, so Allan quickly replaced it with a flue tile.

Strangely enough, while Mr. Brown was designing his wood burner, Dr. Richard Hill of the University of Maine at Orono (a mere 40 miles away from Lincoln Center) was also designing a similar system. Allan hadn’t read about Dr. Hill’s research until after he completed his own unit, but the two men have since been exchanging information in an effort to improve upon both designs.

Since burning wood as fuel is not uncommon in the Pine Tree state, most folks in the area predicted that Allan Brown would have to pay the price of inconvenience in exchange for his savings on fuel oils. But, according to Allan, there’s hardly any bother at all. “I have about 35 acres of beech, maple, and oak that I use for firewood. I have to cut and split it, of course, but I do that in the summer when I don’t mind so much. In fact, I hope to get a full year ahead by next summer. Anyway, after the wood’s split, it’s just a matter of stacking it on the ground floor right near the furnace, ready for the first chilly day. We used about 12 cords of wood last winter without any problems at all. Except for when there’s a real cold snap the firebox will stay hot for eight hours or more. Of course, the ashes have to be shoveled out once every week or so, but even that doesn’t interfere with the heating process because the storage tank holds enough thermal energy to keep the house warm until the job is done, without causing the oil furnace to kick on.

“Needless to say, Allan is more than pleased with the performance of his wood burner … and even happier about the $200 a month he’s saving on fuel oil. Also, the initial cost of his furnace only came out to about $500, though he’s quick to point out that—being an engineer—he was able to get his materials at a lower price than the average person could. Building a similar unit even with salvaged materials, Allan figures, could cost someone else $1,000 or more.

But, even at that price, the furnace would soon pay for itself … and it’s just as effective as a large oil heater (the wood burner puts out about 150,000 Btu’s per hour, as near as Al can figure it). Brown plans on installing a series of solar collectors within his system next summer to cut down on wood consumption. Between these two sources of heat, he and his family should be sitting pretty when winter rolls around again.

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