Creosote, the unburned material that settles out of wood
smoke and accumulates in stoves and chimneys, plagues
everyone who heats with wood. Whether you’re worried
about the possible danger of a chimney (and, perhaps,
house) fire, the detrimental effects that a clogged
stovepipe might have on your heater’s performance, corrosion of the metal in the flue, the inconvenience
and/or expense of chimney cleaning or simply the bad
odor and — when there are leaks in the pipe — the
mess caused by such accumulations, you’d no doubt like to
know as much as you can about how to minimize creosote
buildup.
Consequently, MOTHER EARTH NEWS and Shelton Energy
Research have entered into a cooperative research project
to test three devices which, it’s claimed, reduce the rate
of accumulation of creosote. In this issue we’ll discuss
some of the options open to concerned wood-stove owners and
describe the research project that’s now underway.
Creosote Accumulation
The potential for creosote accumulation arises when
unburned materials in the flue gases — including
vapors, tar mist and soot particles resulting from
incomplete combustion — pass through the chimney. As
the gases cool, the unburned materials can adhere to the
chimney walls. The process is complicated, however, by the
fact that creosote has no single chemical composition,
appearance, density or ignition temperature. Some of its
common forms are tar, flakes, slag, soot and liquid.
The safest and most reliable way to insure that creosote
accumulations don’t become thick enough to cause trouble is
to inspect the chimney regularly, and then clean it when
necessary. After a stove is first installed, the flue
should be checked every week. Then, if the accumulation
rate proves to be slow, the frequency of inspection can be
reduced. The chimney should be swept whenever the deposits
exceed 1/4 inch in thickness.
Chemical chimney cleaners have long been touted as quick,
easy, inexpensive and reliable reducers or eliminators of
creosote. During recent testing, however, it was shown that
if chemical chimney cleaners work at all, they work only
occasionally. Hence, they can’t be relied on to
always keep creosote levels safe.
Another technique that’s commonly suggested for controlling
creosote is to burn an intentional short, hot fire every
day or two. The result of such a practice is either a small
(and therefore supposedly relatively safe) chimney fire
or — more often — a drying and flaking of the thin
tar-like layer with the particles either falling or
being blown out the flue. However, it’s vital that there be
only very thin creosote deposits present when this
procedure is used, and only frequent inspections can
establish that fact.
If the chimney hasn’t been checked or cleaned for along
time — or if the stove has been run at low power for a
few days and a thick, tarry, highly flammable deposit has
built up — an intentional hot burn could trigger a
serious chimney fire. (You should, of course, avoid the
technique if you have any doubts about the safety of the
chimney itself.) In summation, then, though the hot fire
method can work, the numerous “ifs” involved prohibit us
from recommending it.
So, what can be done to prevent, or at least
minimize, creosote buildup in the first place? Two major
factors affect the rate at which the substance collects,
and solutions to the problem are most likely to be found by
dealing with one or both of them: [1] the density of the
smoke going up the chimney (that is, the amount of unburned
material that remains in the gases) and [2] the
temperature of the flue wall (cool walls increase the
condensation and accumulation of creosote).
Flue wall temperatures are affected by the type of chimney
used and by its location. Both
double-wall/packed-insulation and
triple-wall/air-insulation prefabricated chimneys do a
better job of holding flue gas heat than do either masonry
or triple-wall/thermosiphon (air cooled) types. And
any of them will prove more satisfactory if
located inside the house, where the exterior is in contact
with warmer air than would be the case if the chimney ran
up the outside of the dwelling. Long runs of stovepipe
between the appliance and the chimney also decrease flue
gas temperature, and — therefore — increase
creosote accumulation. In fact, the more heat is pulled out
of the fire and flue gases — whether in the stove, in a
heat exchanger accessory, in the stovepipe connector, or in
the chimney — the cooler the smoke will be and the more
the creosote will build up. Thus there’s often a conflict
between improving energy efficiency and minimizing
creosote.
Stove operation, rather than design, is the single
most important factor affecting smoke density in
traditional wood stoves. As discussed in the last issue of
this magazine (see “Wood Stove Smoke“), you can limit creosote buildup by simply burning
only small, hot fires in your stove.
Contrary to popular belief, very dry wood (that
with less than 15 percent 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. Previously, MOTHER EARTH NEWS discussed a number of possibly 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 wood stoves already
installed in homes — would be some sort of retrofit or
add-on device. There are many such aftermarket products
available today,
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/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.
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 wood-burning 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.
Our current tests will, we hope, enable us to measure just
how much less creosote can be expected in a
chimney when a barometric draft regulator is used. In order
to maximize the device’s possible effect, we’re adjusting
it for the minimum draft setting that’s consistent with the
firing rate and heat output typically used in a home
installation.
The Smoke Consumer
The Smoke Consumer consists of a knitted wire mesh filter
mounted under a cast-iron plate. The assembly can be
positioned either at a right angle or parallel to the smoke
flow just as can a simple stovepipe damper. When the
filter is in the closed (across the flow) position, much of
the flue gas moves through its passageways (of course a
little smoke does flow around the device, in the space
between the plate and the stovepipe wall). The manufacturer
claims that the Smoke Consumer reduces creosote by
filtering out particles. Consequently, the filter requires
periodic cleaning: Maintenance includes a recommended
“continuous burn reactor cycle” once a day — which
involves running the stove hot enough to burn material off
the filter — and manual cleaning of the mesh (by
removing it from the unit) weekly.
The Smoke Dragon
The Smoke Dragon is a catalytic afterburner and heat
exchanger that’s designed to ignite any smoke that hasn’t
been burned in the stove itself. The Smoke Dragon’s
catalytic combuster — which is made by Corning Glass
Works — is a ceramic honeycomb structure, about six
inches in diameter and three inches long, coated with a
very thin layer of a noble-metal catalyst (such as platinum
and/or palladium).
Essentially, the catalyst lowers the ignition temperature
of the smoke from around 1100 or 1200 degrees Fahrenheit to about 400 to
600 degrees Fahrenheit. Thus, if the gas is hot enough, and if it
contains adequate oxygen, much of the material will be
burned in and just above the catalyst. Furthermore, once
the catalyst begins working, the smoke temperature can fall
without ill effects because the heat being generated by
the burning gas in the catalyst will warm the incoming
fumes enough so that they too will ignite. The Smoke Dragon
has the potential for reducing creosote, cutting
air-polluting emissions, and increasing the energy
efficiency of the system with which it’s used (much of the
heat released in the combuster is recovered by the heat
exchanger).
However, in practice it’s difficult to predict the
performance of catalytic devices. Factors that could
influence the overall effectiveness of such a unit include
smoke density, oxygen concentration, the degree of mixing
of oxygen and smoke, flue gas temperature, and the amount
of both intentional and unintentional smoke bypass. Thus,
only through testing can the actual effectiveness of
devices such as the Smoke Dragon be determined.
The Testing Procedure
An array of identical stoves — some equipped with
devices and others without — are being operated
simultaneously for 10 days under a variety of firing
conditions. Each product being tested has been installed on
a pair of stoves and two heaters are being run without
devices to serve as controls. By arranging the
experiment in this fashion, we can check on the consistency
and re-producibility of both the devices’ effects and the
test methods.
Creosote accumulation is determined by weighing the “test
portions” of the chimneys before and after the test. These
sections consist of single-wall stovepipe located inside
the laboratory where the controlled climate can give
better experimental control and reproducibility of results.
Using stovepipe instead of factory-built chimney increases
the amount of creosote that accumulates, since the flue
gases cool off more rapidly when passing through standard
stovepipe.
A variety of different wood species and moisture contents
are being used to fuel the stoves over the course of the
experiment, but the same species and moisture
content are used in all six heaters at any given fuel
loading. Furthermore, for each refueling, all six loads
have the same weight within 10 percent, and the total fuel weights
for each stove over a day are the same to within 1 percent.
The combustion air controls on all the appliances are set
to maintain the same burning conditions using the
temperatures of the flue gases just below the devices and
the appearance of the fires as indicators of uniformity. Flue gas temperatures are being
measured both below the devices and above them in the test
sections of the stovepipes. The temperatures are being
monitored constantly, using recorders.
The stoves will be fired for about 10 days at a mixture of
high and low burn rates in order to simulate home
stove use realistically and to assure that any creosote
reducer that performs better under particular firing
conditions gets a chance to show its capabilities. Each
stovepipe test section will then be reweighed. Both “wet”
and “dry” weights will be measured (the sections will be
placed in a 200 degree Fahrenheit oven for roughly three days before
the second weighing). From previous experience we know that
significant amounts of water can be trapped in the creosote
deposits on the flue walls, which could distort the results
if no dry weight were taken.
In our long history of testing stoves and accessory
products, we have encountered many surprises, and we’ve
learned to be careful about prejudging results. The
manufacturers of all three products claim significant
creosote-reducing effects. In our article in the next
issue of MOTHER EARTH NEWS, we’ll report our conclusions,
after having actually put the products to the test.
