Newmac BC 160, Installation, Operating And Service Manual

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required for evaporation, these gases are cooler and more likely to condense than would be the case with
dry wood.
Charcoal may be found more readily if the unit is 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 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 or 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 150 ºF. 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
boiler 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 highest 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 severe 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, stoking 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 still 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
accessory 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 stove pipe 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 stove pipe 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