Creosote And Chimney Fires - Newmac WAO E Installation, Operating And Service Manual

Does green wood cause creosote?
Yes. Indirectly, green wood does cause creosote. The exhaust gases cool as they rise up the chimney. If
the temperature falls below the dew point, any moisture contained in these gases will condense on the
inside of the chimney, absorb the various products of incomplete combustion and form creosote. When
green wood is burned, the exhaust gases carry a high moisture content in addition, because of the heat
required for evaporation, these gases are cooler and more likely to condense than would be the case with
dry wood.
Charcoal may be formed more readily if the unit is overcharged particularly in milder weather. With
overcharged particularly in milder weather. With overcharging (too much wood in unit) the draft fan will be
off a greater percentage of time, coals will be formed which will become covered with ash in turn will
smother the coals to form charcoal. Any coals in the furnace should be stirred before more wood is
added to it.
The preceding is an expert 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
o
150 F. The condensation 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 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 adhere strongly to a stove pipe and
thus are easy to brush off; some of the other forms will not budge 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 mush as desired. This will promote complete combustion during pyrolysis, when the creosote
compounds are being formed, but there will be significant heat surge while the gases are burning.
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