Testing Creosote-Removing Devices

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Contrary to popular belief, very dry wood (that with less than 15% moisture content) usually increases creosote accumulation in stoves. In open appliances such as fireplaces, however, the use of green wood usually increases creosote accumulation. Pitchy pines have long been considered to be heavy creosote producers, but the effect isn't always very marked.

Appliance design can also influence the accumulation of creosote. In issue 72, MOTHER discussed a number of possi bly relevant design features . . . including secondary combustion, catalytic combustion, and the high turbulence furnace. These approaches are not always effective (good ideas require good engineering), but each of them does have promise.

Perhaps the most appealing option for reducing creosote—given the large number of woodstoves already installed in homes—would be some sort of retrofit or add-on device. There are many such aftermarket products available today (some of them were described in issue 72), but—despite their popular appeal—little if any scientific evidence exists to show whether any of the devices actually work.

In the Shelton Energy Research/THE MOTHER EARTH NEWS® cooperative research project, three retrofit products are being tested: a typical barometric draft control (we're using one made by Steinen of Carolina), the Smoke Dragon catalytic afterburner, and the Smoke Consumer. (All three are represented in the accompanying illustrations.)

BAROMETRIC DRAFT CONTROL

Barometric draft regulators are designed to prevent excess draft, and are usually installed in the stovepipe between the appliance and the chimney. (Such devices can't, of course, help a chimney with inadequate draft.) Barometric draft controls are equipped with a hinged and weighted flap that's closed when there's no fire in the stove. During use, however, when the draft in the chimney exceeds a preselected value, suction pulls the flap open. This lets room air into the chimney, thereby preventing the draft from becoming greater than the chosen setting (adjustments are made by moving the weight attached to the flap).

Barometric controls, by limiting draft, also limit the intensity of the fire. This results in a steadier heat output and protects the stove and chimney from overheating. (Such functions are usually more important for coal-fired than for wood-fired appliances, but they come into play with large central woodburning furnaces too.)

The important issue in this study, however, is the creosote-controlling potential of barometric draft regulators. Added air both cools and dilutes the smoke in the chimney. The dilution air also increases the total flow up the chimney (despite the fact that it lowers the temperature of the gas), and therefore yields higher flue gas velocity. Though the net effect of all these influences is hard to predict theoretically, studies done at Shelton Energy Research—using devices other than barometric draft controls—have shown that the introduction of dilution air can dramatically reduce creosote accumulation. [EDITOR'S NOTE: Information on obtaining SER's research report on dilution airand on other projects—is listed at the end of this article.]

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