Wood Stove Efficiency and Emissions: 1984

Research shows wood smoke has more pronounced health effects than previously understood, and that wood stove efficiency plays a large role in stove emissions. If you’ve been putting it off, the time may have come to buy a better wood-burning heater.

| September/October 1984

  • wood stove efficiency - illustration of a stove expanding and contracting
    The addition of catalytic combustors has not only improved wood stove efficiency, they've cut down on the noxious clouds those old cast iron behemoths used to huff out.
    Illustration by MOTHER EARTH NEWS Staff

  • wood stove efficiency - illustration of a stove expanding and contracting

One of the quickest and most convincing ways to learn the true worth of a gallon of oil is to cut enough firewood to duplicate its heating value. In raw Btu, it takes about 20 pounds of wood to equal a gallon (about 7.5 pounds) of fuel oil. Imagine pumping a winter's worth of fuel oil up out of the ground, toting it home in quart bottles, and pouring it into a heater every few hours! As appalling as that prospect sounds, people who burn wood perform a ritual that's very similar — except that they haul twice as many pounds every winter.

Contemplating the amount of labor that goes into heating with wood can be depressing, but what really hurts is poor wood stove efficiency: about half of the energy available in the fuel goes to waste. Worse yet, much of the 50% that gets away pollutes the air or sticks to the inside of the chimney.

The trade-offs of heating with wood have been a fact of life for decades. And until the question of pollution came up a few years back, few wood burners considered it a bad deal. With a moderate amount of honest labor, anyone who burns wood can save on his or her heating bill. And trees are a renewable source of energy. The alternatives — oil, gas, and electricity — are more expensive than wood and are becoming even pricier as the supply of fossil fuels is depleted. Furthermore, fossil and nuclear energy sources can't lay claim to great efficiency themselves. By the time usable heat is delivered by most conventional energy sources, at least half of the Btu have escaped. And, just as is the case with wood burning, pollution is one of the major by-products of inefficiency. Therefore, we wood users must ask ourselves two questions about this tradeoff: Just how bad is the problem of pollution from wood stoves, and what can be done about it?

Smoke Gets In Your Eyes

In some locales, wood stove pollution is quite serious, constituting the major source of particulate emissions. And on a national basis, wood burning produces a significant share of some very dangerous compounds. According to Dr. Dennis Jaasma, a wood-combustion research scientist at Virginia Polytechnic Institute and State University, the most threatening pollutants are particulates, vapor-phase hydrocarbons, and carbon monoxide. All of these substances can be drawn deep into the lungs through normal breathing, and thus pose various health hazards. Scientists don't know for sure what the annual national contribution of wood burning may be to the emission of these compounds, but estimates have been made that the numbers are about 0.7%, 2.5%, and 4%, respectively. (By comparison, diesel engines contribute about 1% of the national annual particulate emissions and are regulated by the Environmental Protection Agency.)

But it's actually a subcategory of particulates and vapor-phase hydrocarbons that may be of the greatest concern. Many polynuclear aromatic hydrocarbons (PAH) are known to be mutagenic and carcinogenic, and it has been estimated that wood stoves emit about 36% of the total annual load of these compounds in our atmosphere. However, there is great uncertainty in this estimate, because not enough research has been done. What's more, the testing procedures for measuring PAH are expensive, complicated, and of dubious accuracy. Jaasma believes the uncertainties are such that the PAH contribution from stoves could lie anywhere in the 5 to 75% range. In any event, this makes wood stoves a major source of PAH.

Revulsion = Revolution

The presence of smoke is blatantly obvious to anyone living just downwind from the chimney of an active wood stove, and regulatory agencies have taken notice of what comes from such flues. In Oregon, the sale of "dirty" wood-burning space heaters will be banned as of January 1, 1986, and in 1988 the rules will become significantly more strict. Colorado has passed a law that may result in regulations patterned after Oregon's, but it will also cover fireplaces by requiring certain design features. Up in Missoula, Montana, episodic wood stove pollution has become so severe that there are prohibitions on burning during alerts, and an outright ban on wood burners has been considered.

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