Creosote And Chimney Fires - Newmac WG-100 Installation, Operating And Service Manual

The preceding is an excerpt from a document prepared by the Nova Scotia Energy Council and the Nova
Scotia Research Foundation Corporation.

CREOSOTE AND CHIMNEY FIRES

Wood combustion is never perfectly complete. Wood smoke almost always contains some unburned
gases and a fog of unburned tar-like liquids. Some of these materials will condense out of the flue gases
onto any surface which is not too hot. The condensate is usually dark brown or black, and has an
unpleasant acrid odor. It is called creosote. If condensed on a relatively cool surface (such as an exterior
stovepipe chimney), the creosote will contain a large amount of water along with the organic compounds,
and will thus be very fluid. Water is usually absent if the condensation occurs on surfaces hotter than
0
150 F. The condensate may then be thick and sticky, like tacky paint or tar. Creosote may be found almost
anywhere in a wood-heating system, from the top of the chimney to the insides of the cover itself.
Creosote which remains in a chimney after its initial formation may later be significantly modified both in
physical form and chemical content. The water and the more volatile organic compounds tend to
evaporate, leaving the more tar-like substances behind. If these are subsequently heated by the flue gases
from a hotter fire (this usually happens), they themselves are further pyrolyzed to the same final solid
product that wood is carbon. The physical form is usually flaky, and often shiny on one side. Partially
pyrolyzed deposits can have a bubbly appearance. The flakes do not adhere strongly to a stove pipe and
thus are easy to brush off; some of the other forms will not budge even under the action of a stiff wire brush.
The amount of creosote deposited depends mostly on two factors – the density of the smoke and fumes
from the fire, and the temperature of the surface on which it is condensing. Highest smoke densities occur
when a large amount of wood in relatively small pieces is added to a hot bed of coals and the air inlet
damper is closed. Here, there is considerable pyrolysis of wood, but little combustion, and little air to dilute
the smoke. In practice, creosote generation is higher during low-power, overnight, smoldering burns.
Smoke densities are least when combustion is relatively complete, which tends to be the case when the
amount of excess air admitted to the wood-burner is high. Leaky stoves, open stoves and fireplaces
typically have the least creosote problems.
One way to lower the average smoke density in an airtight stove is to use less wood each time fuel is
added, and/or to use larger pieces of wood. In either case, the air supply need not be turned down so
much in order to limit the heat output and combustion is likely to be more complete. Of course, if less wood
is added, stokings must be more frequent. A related procedure to limit creosote is to leave the air inlet
moderately open after adding wood until the wood is mostly reduced to charcoal, and then close the inlet as
much as desired. This will promote complete combustion during pyrolysis, when the creosote compounds
are being formed, but there will be a significant heat surge while the gases are burning.
Extra air can also be added to the flue gases in the stove pipe; this is what the Ashley creosote inhibitor
does. But the net effect of adding dilution air is not obvious or necessarily beneficial. Dilution air will
decrease the smoke density, but it will also decrease its temperature. These effects have opposing
influences on creosote formation. The National Fire Prevention Association states that dilution air
increases chimney deposits. In any case, the cooling effect of dilution air does decrease the heat transfer
through the stovepipe and chimney, thus decreasing the system's energy efficiency.
Creosote formation may also depend on the type of wood burned and on its moisture content. Dry
hardwoods have a reputation for generating the least creosote, but the quantity can still be very large. No
kind of wood eliminates creosote formation.
For a given smoke density near a surface, the cooler the surface, the more creosote will condense on it.
The phenomenon is very similar to water vapor condensing on the outside of a glass of ice water on a
humid day, except for an inversion – condensation occurs on the inside of a chimney, especially when cold
air outside makes the inner chimney surface relatively cool. A stovepipe chimney outside a house on a cold
day will be wet on the inside with creosote (including a lot of water) virtually all the time. A well insulated,
pre-fabricated metal chimney has the least serious creosote problems; its insulation helps maintain higher
temperatures on its inner surface, and its low heat capacity allows it to warm up very quickly after a fire is
started. Masonry chimneys frequently accumulate deposits at the beginnings of fires and their interior
surfaces take a longer time to warm because the construction is so massive. Any type of chimney which
runs up the outside of a house is more susceptible to creosote problems than the same type of chimney
rising in the houses' interior, due to the cooling effect of the colder outdoor air on the exterior chimney.
Average flue gas temperatures can be increased by minimizing the length of stovepipe connecting the
stove to the chimney. This, of course, will also decrease the energy efficiency of the system, and it is often
true that measures which decrease creosote formation also decrease heating efficiency. For instance,
stoves which have energy efficiencies due to their relatively good heat transfer (e.g. the Sevca, lange 6303
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