Wood Stove Safety

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 burnthem, 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!