Wood Stove Safety

Though there's a long way to go in categorizing and measuring all the effluents involved, research done in the past year has produced a wealth of new information on wood smoke and wood stove safety.

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Wood stove safety research in 1980 found wood stove smoke contains polycyclic organic matter and other harmful substances.


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Ever since the first alarms about wood stove emissions were sounded, almost two years ago, there's been a flurry of research done on the subject of wood stove safety. Many of the findings of the spring-of-1980 Monsanto study that formed the groundwork for concern have been supported. But some of the new data have led to questions about the relevance of the testing methods that the Monsanto/Auburn University researchers used to reach their startling conclusions.

Pollution Today

Work done by more than a dozen different laboratories has confirmed that "airtight" wood stoves do emit large amounts of carbon monoxide, particulates, and unburned hydrocarbons. But the importance of the Monsanto team's discovery of significant amounts of "polycyclic organic matter," or POM (sometimes called "polycyclic aromatic hydrocarbons", or PAH ... many of which are known carcinogens), in smoke is now in doubt.

Throughout the course of its research, the Monsanto/Auburn group maintained burn rates of 15 (or more) pounds of wood per hour; most homeowners fire between two and six pounds of wood per hour. It was assumed at the time that the lower, more common burn rates would yield even greater quantities of POM than those noted in the tests since hydrocarbon emissions—in general—increase as wood consumption drops. But work done in the last year has failed to confirm that theory. Though scientists emphasize that the new data are far from complete or conclusive, the emissions of some POM compounds seem to drop at the lower burn rates typical of home wood stoves. Argonne National Laboratories tests, for example, have found little evidence of benzo (a) pyrene (a particularly toxic PAH) at typical household firing levels.

However, before we breathe a possibly premature sigh of relief (and, perhaps, inhale something that we might later regret), we must accept the fact that there's a great deal left to be learned about wood stove smoke. For example, a recently released report of the preliminary findings in a Dow Chemical study suggests that dioxins (see "Bonnie Hill: Oregon Environmental Activist" for more information on these extremely hazardous compounds) may be produced in woodburning appliances. (The cynical among us might question Dow's motives in conducting this bit of research, however, since the report concluded that the offending toxins did not come from herbicide-contaminated wood.).

Perhaps the newest and least understood of the woodburning-related pollution problems came to light with the discovery (by a Geomet Technologies team) that many wood stove-equipped homes have indoor levels of carbon monoxide, breathable particulates, and even members of the POM family that are more than ten times greater than outdoor measurements taken at the same time. Unfortunately, it'll be a while before we learn just how such a predicament occurs, since the financially gutted Environmental Protection Agency has no funding to build on the work done by Geomet.

In fact, our understanding of the elements and compounds that occur in wood stove smoke is far from complete. Scientists have been studying automobile and coal-fired powerplant emissions for decades, but we're just beginning to get an idea of the possible environmental impact of woodburning heaters.

The Silver (or at Least Shiny) Lining

There is also a positive side to our recent wood stove emissions research. We've learned more about the physics of combustion in the last couple of years than in all the years since humankind discovered fire. In turn, new stove designs and aftermarket products are rapidly being developed to deal with already recognized pollution problems. In addition, a wealth of information concerning how a wood stove owner can operate his or her stove in a cleaner fashion has recently become available.

And there are fringe benefits to the pollution studies, as well. Creosote—that scourge of every woodburner—is produced under the same conditions as are pollutants. The gooey substance that clings to stovepipes is created by unburned material expelled from the fire below. Though the actual ratio of creosote deposit to emissions varies (depending on factors such as flue gas velocity, stack temperature, etc.), more pollution is almost always accompanied by heavier creosote formation. (According to Dr. Stockton Barnett of the State University of New York, an average stove installation leaves about 10% of its particulate and hydrocarbon emissions in its flue, in the form of creosote. )

What's more, the majority of the schemes devised to reduce emissions and/or creosote buildup also tend to improve the overall efficiency of wood stoves. Since most of the materials that make up smoke happen to be combustible, a popular approach to preventing the offenders from reaching the atmosphere is to burn them, either in or above the fire. And, of course, doing so also increases the amount of heat that can be gotten from a given amount of wood!

Operator Techniques for Cutting Emissions

Despite the fact that the study of woodstove emissions and efficiency is in its infancy (and, therefore, different groups still use divergent test methods and arrive at sometimes conflicting conclusions), researchers are unanimous about one facet of the pollution/creosote production process: Every study has found that operator methods can play a larger role in reducing creosote and emissions than can any other factor that's yet been examined! As a result, a clear-cut set of clean-burn instructions has been developed over the past year.

The five rules which we're about to offer are mostly matters of common sense. But in order to understand why they make sense, you'll need to know a little about how wood burns. The combustion process has been theoretically divided into three phases: evaporation, where the moisture in the wood is removed; pyrolysis, the release of volatile gases trapped in the fuel's structure; and charring, during which the material's carbon (in the form of charcoal) is burned. However, as systematic and neat as this outline sounds, it is complicated somewhat by the fact that the different stages almost always overlap. Only at the very beginning and end of a burning cycle (early evaporation and final charring, respectively) are the distinctions clear.

The emission of particulates, hydrocarbons, and carbon monoxide is primarily the result of two different situations that can arise during the phases of burning. In the pyrolysis stage, either a lack of oxygen or inadequate temperature (it must be 1100°F or more) above the fire will allow the volatile gases to escape without burning. Any of several factors could lead to either of the conditions. For example, if the stove's draft is too far closed, there may not be enough air to burn the pyrolytic products.

Emissions can also result from incomplete combustion in the charring stage. And, again, the culprits here are too little air and/or heat. Oxygen starvation might result from a closed down draft or inadequate separation of the individual pieces of fuel, which can prevent air from reaching the burn area. Heat can also be lost in other ways, one of which is the addition of new wood to a bed of coals. The evaporation of the moisture in the new fuel can draw large amounts of heat from an already established burn.

Of course, the list of factors that could trigger either of the two major pollution producing combustion situations goes on and on, but the rules of thumb that follow have proved—by actual experimentation—to deal with many of them.

Five Ways To Clear the Air 

Rule 1: Use the largest-diameter logs that will burn effectively. Big pieces of wood have less surface area per unit of volume, which prevents them from releasing volatiles too rapidly. This has been recognized as the single most effective technique for reducing emissions!

Rule 2: Build as small a fire as is practical. A stuffed firebox often leads to areas of pyrolysis and/or charring that can't be reached by an adequate air it supply. Therefore, use as few of the large pieces of wood as you can while producing adequate heat.

Rule 3: Keep the fire hot. Position the logs in your stove so that air can move through the fire zone, and be sure there's sufficient draft opening. Since you're already trying to make the fire as small as possible, you can maintain high temperatures inside the stove without overheating your home.

Rule 4: Don't increase or decrease the draft setting drastically. Pyrolysis continues for some time after the air supply has been cut back; so slamming the damper shut can send much of your hard-won fuel up the chimney. On the other hand, rapid opening of the damper can carry the pyrolytic products away from the fire too quickly, especially if there's a significant wind-induced draft.

Rule 5: Avoid excessively wet, or dry, wood. Logs that are too dry pyrolyze very quickly, overloading the combustion zone with volatile gases. Moist timbers can inhibit effective combustion by absorbing heat for evaporation. Standard air-dried soft or hard firewood (with about 20 to 25% moisture content) seems to be the cleanest-burning fuel.

Naturally, in order to observe these five rules, some stove owners will have to change their habits slightly. Heaters will require loading more frequently than was the case during the era of the all-night burn. But, on the positive side, many of us won't have to split logs as thoroughly as we have done. And we must emphasize that following the procedures listed above will not only cut down the pollutants coming from your wood stove, it'll also help to keep your stovepipe cleaner and allow you to obtain more heat from a given amount of wood.

Is There a Cleaner Stove?

Of course, it would be nice if we could just go out and buy clean-burning stoves and reap the added benefits of clean flues and greater efficiency. Unfortunately, among the standard airtight heaters—whether they be updraft, downdraft, sidedraft, baffled, cast iron, ceramic, or whatnot-only the thin—walled steel stoves seem to show any repeatable difference in performance: Such devices consistently produce slightly more pollutant material and are slightly less efficient than most conventional stoves.

However, we're about to describe a number of unconventional designs that do achieve a significantly cleaner burn than others.

Burned Again 

One widely explored technique for cutting emissions and increasing efficiency involves encouraging secondary combustion. With this approach, specific provisions are made to encourage the burning of pyrolytic products away from the fire. Many modern stoves have secondary draft controls that are designed to introduce combustion air to the secondary zone. If such combustion actually took place with any frequency, these appliances would burn considerably cleaner than they actually do. Unfortunately, most researchers have noted that it's extremely difficult to establish and maintain a secondary burn with anything less than "wide open" fires. In fact, among the conventional heaters equipped with secondary air inlets, it is fair to say that such "afterburning" rarely occurs.

Scientists and engineers still believe, however, that establishing a secondary burn zone is a potentially viable way of accomplishing the goals of higher efficiency and reduced emissions, and much design work has been, and is being, dedicated to perfecting the approach. One company involved in this research is Jøtul of Norway, which has been working with both catalytic devices and secondary combustion for almost six years. In February of 1982 Jøtul will introduce its Model 201, which will include a secondary combustion system that preheats air to around 750°F.

The company's new stove is similar in concept to the wood gasifier that powers MOTHER EARTH NEWS' pickup truck, and its efficient use of the pyrolytic products promises to yield high efficiency. Jøtul's own testing has shown combustion efficiencies of between 92% and 98% over a wide range of burn rates, though the high secondary combustion temperatures involved can result in enough stack losses to reduce overall efficiency to between 76% and 80% (still very impressive figures).

From a pollution and creosote control standpoint, the 201's high combustion efficiency should cut CO, hydrocarbon, and particulate levels effectively. While Jøtul is unwilling to release full performance figures, the company has stated that the stove's CO output will be less than 0.3% (Switzerland has the most stringent current regulation, allowing levels no higher than 1%). At a suggested retail of under $1,000, the 201 could prove to be a powerful tool in the fight against emissions and creosote.

Another method of encouraging secondary combustion is used in the Brugger Industries (of New Zealand) wood stove. The Brugger's secondary air tube runs directly above the fire before exhausting below a series of holes in a baffle between the firebox and the flue. According to the manufacturer, the system leads to the ignition and continued consumption of volatile gases at burn rates as low as 1 1/2 pounds of wood per hour.

An interesting development from Essex Thermodynamics Corporation carries the secondary combustion concept to its logical extreme. The Essex boiler is actually a downdraft wood gasifier, in that it draws intake air down through the fuel and creates burnable pyrolytic products as one of its major sources of heat. The volatile gases are pulled through the coals at the bottom of the bed (to enrich the mixture with CO and hydrogen) and are then fired in a ceramic-lined combustion chamber. The heat is passed on to a water-circulation system through a series of parallel tubes.

Several other gasification-type stoves are currently in the works, and the Oregon Department of Environmental Quality has performed some preliminary testing on one of them. The unit's emission levels were somewhat lower than those of conventional heaters, but were greater than those produced by the high temperature furnaces we'll discuss later in this article. (Oregon's researchers did note some problems with fuel bridging in the gasifier. If these are corrected, that stove's performance could improve.)

Catalyst (On a Hot Tin Stove)  

Scores of stoves with catalytic combusters have appeared in the marketplace during the last year, as manufacturers have attempted to provide consumers with salable technological improvements. The honeycomb ceramic and noble metal (platinum, for one) devices work by initiating combustion at a lower temperature than would normally be possible ... without being consumed themselves. A typical converter begins affecting the oxidation of carbon monoxide at around 500°F. As temperatures rise, it helps to ignite heavier combustibles as well. In practical terms, a catalytic device makes secondary combustion occur at temperatures that are between 300° and 600°F lower than would normally be possible.

The performances of a number of different stoves fitted with Corning's Catalytic Combusters are now fairly well documented. In that testing, reductions in emissions and increases in efficiency have been found at medium-to-high burn rates (those of more than about five pounds per hour). But the effectiveness of the catalyst system at low burn rates isn't nearly as well understood. Some researchers have found that the combusters won't "light-off" at reduced firing levels. Which, under normal home woodburning conditions, would make such stoves little (or no) more effective than conventional heaters. However, 1981-82 will be the first full winter during which consumers will be using such appliances, and much should be learned from the public reaction to in-home use of catalytic combuster-equipped wood stoves over the course of this winter.

Burn Now, Heat Later 

The high-temperature furnace is probably the most thoroughly proven low-emissions woodburning approach. Dr. Richard Hill, of the University of Maine, was the designer of the original high-turbulence (as it's often called) furnace. It achieves excellent combustion efficiency and low emissions by burning wood at an extremely rapid rate in a well-insulated ceramic firebox with a forced-air intake. The fuel is quickly consumed, and the heat is absorbed by water (in the case of Dr. Hill's system) for storage.

The emission levels of such furnaces are quite low. They are, in fact, comparable to those of burners fueled by oil (though they're significantly lower in sulfur content). Overall heating efficiencies of the furnaces, however, are not much higher than those of conventional stoves, since some heat is lost during the exchange and storage stages.

Add-On Equipment

As is the case in the automobile fuel mileage gimmick business, there are a lot of would-be inventors—in the woodburning industry and out—who'd love to come up with something that a person could stick on the outside of an existing stove to reduce emissions and/or creosote. So far, a number of products have appeared which are said to do just that, but accurate testing lags far behind the claims. Unless otherwise noted, we're not suggesting that the following devices perform as the manufacturers say they do. We're only trying to inform you of their existence.

Temperature Control  

There are several items on the market that assist the wood stove owner in monitoring the operating temperature of his or her appliance. Such products can help a woodburner comply with Rule 3 for clean operation: Keep the fire hot. Furthermore, when one has a thermometer to watch, it's an easy matter to keep track of heat output from the comfort of an armchair. The simplest and least expensive stove thermometer is the Chimgard bimetal coil device made by Condar Company, and sold through many wood stove supply stores. The Chimney Fire Alert by Vermont Technologies Group, Inc.  is a more sophisticated temperature sensor that consists of a thermocouple (which is inserted through the stovepipe wall) and a remote control box. The device includes a thermometer, but its major feature is an adjustable alarm that signals overheating or a chimney fire.

Yet another approach to maintaining consistent (and high) operating temperature in a wood stove is the thermostatic air intake control. Many commercially available stoves come equipped with such mechanisms, but—in general—the units respond too slowly to maintain very even stove temperature. The lag time in the reaction of the typical bimetal coil causes many thermostatically controlled stoves to oscillate around the desired temperature. (You may have seen the chimney of a heater equipped with such a device emitting puffs of smoke as if the occupants of the house were sending signals.)

However, a new thermostatic intake air regulator—developed by Stockton Barnett, for marketing by the Condar Company—has been designed to react very quickly to changes in stove temperature. Test results indicate that the Stovetemp is able to maintain very even temperatures in wood stoves, and thus to eliminate the inefficient over- and underfiring common to most heaters. Dr. Barnett didn't measure any appreciable drop in the amount of hydrocarbons and particulates emitted, per pound of fuel burned, in a selection of Stovetemp-equipped stoves ... but the improved efficiency of the appliances (which typically ran between 13% and 20%) caused less wood to be burned in order to warm the homes, a condition that—in turn—leads to reduced emissions overall.

Draft Inducement 

Several different techniques have been developed to encourage proper draft in wood stove flues. There are a number of chimney caps which—according to their manufacturers—increase flue gas velocities and thereby reduce creosote formation. (One such firm claims that its product will cut creosote accumulation by as much as 75%.) Many advertisements also state that chimney caps may improve combustion efficiency, which could—as an indirect effect—also result in lower emission levels. This is still very much in question, however, since an increase in flue gas velocity (which might reduce creosote accumulation) wouldn't guarantee that emissions would drop, too.

Barometric draft regulators have also caused a bit of a stir in the marketplace over the last year. The devices are—as you'd expect—similar in concept to oil and gas furnace regulators. As flue gas velocity rises, a damper opens to admit air into the chimney, which prevents overburning caused by excessive draft and adds dilution air to the stove's exhaust. As we mentioned in reference to chimney caps, barometric draft regulators may well reduce creosote accumulation (our research project, being performed in conjunction with Shelton Energy Research, is exploring that question right now), but a lowered rate of emissions may not in this case go hand in hand with a cleaner chimney.

It's also worth noting that barometric draft regulators—if ducted from air inside the house—may increase the overall number of air changes in a house ... which would, in effect, lower a stove's overall heating efficiency. Furthermore, we believe that draft regulators should be conveniently located and equipped with a manual override, allowing them to be shut in the event of a chimney fire.

Add-On Afterburners 

Two companies now sell catalytic combusters as retrofit devices for wood stoves. Energy Harvesters is offering a stove with what is termed the "Energy Cat" option, and the setup can be added to the company's older models as well. The Smoke Dragon (a product of Penn Stove), on the other hand, can be used with many different brands of wood stoves. However, at $350, this Corning Combuster-equipped unit will add about as much to the price of a wood stove as you'd expect to pay in supplemental charges for a factory-installed system. If its performance is similar to that of the other catalysts that have been tested, the Smoke Dragon should reduce emissions and increase the overall efficiencies of conventional wood stoves. (The Penn Stove product is one of the devices MOTHER EARTH NEWS and Shelton Energy Research are testing.)

The Smoke Consumer—another retrofit product—is an add-it-to-your-stovepipe module that attempts to accomplish goals similar to those sought by catalyst add-on manufacturers, but aims to do so by different means. Lincoln Works' product captures particulates and hydrocarbons on a fine-mesh stainless steel filter. Material which would have become either creosote or pollutants is thus caught as it passes through the convoluted passageways (the device is similar in concept to an automobile air filter), and—if the exhaust gas temperatures are above 300°F—the residues will be burned away either on the filter or on the cast-iron reactor plate above it. However, when the temperatures are lower than 300°F, deposits will form that either must be burned away (by a daily hot firing) or cleaned off. Consequently, the Smoke Consumer's manufacturer recommends that the screen be inspected at least once a week, and cleaned when necessary.

Though Lincoln Works doesn't make any claims about its product's ability to reduce emissions, it could affect both particulate and hydrocarbon output. In addition, it's possible that carbon monoxide levels might also be cut by the filter. We'd like to emphasize, however, that these are mere speculations on our part. (The Smoke Consumer's effectiveness as a creosote-reducing device is being examined in the Shelton Energy Research study.)

Burn Less Wood

In conclusion, it should be said that the best method we know of for lessening wood stove pollution problems, cutting back on creosote, and easing the burden on our nation's forests is to reduce our need for heat. More and more folks are switching to wood fuel for its economic advantages, and in many parts of the country the load on the environment—in the form of pollution and deforestation—has already become critical.

So before you buy a bigger stove or head out to bring in another cord of wood, consider the possibility of weatherizing your house. There's a good chance that adding insulation, sealing cracks, and putting up storm windows may prove to be less expensive—in the long run—than burning an extra grove of trees. And give some thought to installing solar devices as well. Buying or building them will cost dollars and energy, but once they're in place, the sun gatherers will usually require a minimum of attention for years to come.

The lessons that we're now learning about wood stove pollution are actually a reiteration of the lessons we've already' been taught by coal, oil, gas, and uranium: Too much of anything can be a problem. In the long run, there's no substitute for doing more with less!