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Cleaning up Woodstove Heat Emissions

A discussion on cleaning up woodstove heat emissions for 1982-1983, includes an introduction to woodstoves, the condar stovetemp thermostat, stack temperature woodstove thermostat and how to build the thermostat.

| September/October 1982

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    Our experimental setup on a Buck Stove. The sensor line leads down the back of the stovepipe.
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    Diagram: Thermostat parts layout.

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With the chill of autumn rapidly settling in and woodpiles beckoning, it's time to look at cleaning up wood heat emissions for 1982-1983. 


Last autumn, when we printed our article "Woodstove Smoke" (see MOTHER EARTH NEWS issue 72, page 72), we were able to report on a number of interesting developments that — it was hoped — would help solve the related problems of woodburning emissions, creosote, and efficiency. Since then, many of the experimental technologies discussed in that story have produced readily available items . . . and an interest in marketing clean, efficient products seems to have penetrated the mainstream of the wood-heating industry.

In the hurry to get new stoves into stores, however, the industry's research — though still extensive — simply hasn't born as much fruit this year as it did last. Still, we'd like to use this heating season's report to update the developments mentioned in last year's story . . . tell you about a few interesting new items on cleaning up woodstove heat emissions . . . and detail some work we've been doing ourselves!

Woodstoves and Secondary Combustion

As you may remember, woodstove smoke can be cleaned by a system that burns it outside the primary firebox. Secondary combustion — as the process is called — has been under study for several years now . . .and a number of products are, in 1982, using it in attempts to improve efficiency and reduce pollution. The most fanfare thus far has gone to Jøtul's new model 201, which was introduced in February. Independent laboratory testing has confirmed the Norwegian company's claims of an overall efficiency between 75 and 80% (as compared to a conventional Jøtul's rating of 55 to 60%). The small heater is now available, at Jøtul dealers, for less than $1,000.

Several other residential space heaters, and one woodburning boiler, are also using secondary combustion or wood gasification (which could be seen as the same process carried to extremes). Unfortunately, many of the companies manufacturing these appliances are small and can't afford the expensive testing which would serve to confirm or deny any claims for the success of their designs. (Some of our staff members, however, have seen the Essex Thermodynamics boiler in operation . . .and were quite impressed.)

Woodstove Catalysts

A catalytic combuster is a device that causes secondary combustion to occur at lower temperatures than would ordinarily be possible. The honey-combed cells, which can be designed into conventional stoves or even added — as aftermarket items — to existing models, have been popular with manufacturers because the relatively simple devices offer some efficiency improvement for a minimum of retooling cost. Consequently, there are better than 20 companies selling catalytic combuster-equipped products.

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