Newmac CL 86-96C Installation, Operating And Service Manual page 35

Green or wet wood is undesirable for several reasons. Green or wet wood tends to mildew and rot which causes a significant
reduction in the thermal value. When green or wet wood is burned, it may take 20 to 25 percent of the thermal value of the
wood heat to evaporate and drive off the moisture which is contained. Green wood does not burn easily and, in order to keep
the fire burning, it is often necessary to add a lot of fuel and provide excessive draft, thereby decreasing the efficiency of the
unit. The excess air needed for combustion must be heated and it escapes up the chimney wasting heat that should be used to
heat the house.
What is creosote?
Wood smoke almost always contains some unburned gases and a fog of unburned tar-like liquids. Some of these materials will
condense on the inside of the chimney, just as steam condenses on any cold surface. This condensation is a black, tacky, fluid
when first formed. When it dries, it is flaky and shiny. Creosote has approximately the same thermal value as fuel oil. Not only
does it reduce the effective size of the chimney, but an accumulation of this material constitutes a serious fire hazard.
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 overcharge 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 condensation 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 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 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, 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 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.
34