Newmac CL 86C Installation, Operating And Service Manual page 37

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, prefabricated 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. Nay 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 and double barrel stoves) are
more likely to have chimney creosote problems precisely because they do such a good job extracting heat from
the flue gases.
Generally creosote is inevitable and must be lived with. Any kind of chimney deposit decreases the system's
heating efficiency. Soot and dried creosote accumulations have a significant insulating effect; less of the heat in
the flue gases transferred into a house through dirty stovepipes and chimneys. The most annoying problem can
be creosote dripping from a stovepipe or chimney, and the most dangerous problem is chimney fires, during
which the creosote, or its pyrolyzed residue, burns.
Creosote dripping can usually be eliminated. Joints in vertical segments of stovepipe will not leak if, at the joints,
the smaller, crimped ends always stick down into the receiving end. (Smoke will not leak out of the joints due
to this direction of overlay.) Since this is not the usual orientation for stovepipe, a double male fitting may be
necessary at some point to connect the stovepipe to the stove, a prefabricated chimney, or a rain cap. Special
drip proof adapters are available for connecting some sizes of stovepipe to Metalbestos brand prefabricated
chimneys. Common types of stovepipe elbows can leak creosote due to their swivel joints; rigid and accordion
type leak proof elbows are available. Horizontal or gently sloping joints between horizontal pipes and/or fillings
are the most difficult to seal against dripping. A good high temperature sealant can sometimes help, but is no
guarantee. The joint must also be snug, and well secured with sheet metal screws. If all joints are made leak proof,
then the creosote will generally drip into the stove, where, when the fire is hot, it will be burned.
Chimney fires occur when the combustible deposits on the inside of a chimney burn. The deposits may be
'raw' creosote, pyrolyzed creosote, or soot. Ignition requires adequate oxygen, which is usually available, and
sufficiently high temperatures - the same conditions as for the ignition and combustion of any fuel. Chimney fires
are most likely to occur during a very hot fire, as when cardboard or Christmas tree branches are burned, or even
when a stove burns normal wood, but at a higher than normal rate. A crackling sound can often be heard at the
beginning of a chimney fire. As the intensity of the fire rises, the stovepipe will sometimes shake violently, air will be
very forcefully drawn in through the stove, and the stovepipe may glow red hot. A tall plume of flame and sparks
can be seen rising from the top of uncapped chimneys.
The most effective way to suppress a chimney fire is to limit its air supply although both water and salt are sometimes
suggested if a relatively airtight stove is the connected appliance. This is easily done by closing the stove's air-
inlet dampers, if all the stovepipe and/or chimney joints are tight, and if no other appliance is connected to the
same flue.
In a properly designed and maintained chimney, the only potential hazard related to chimney fires is ignition
of the building's roof or surroundings due to sparks and burning embers coming out of the top of the chimney.
A spark arresting screen can decrease, but not eliminate this possibility, but spark screens themselves are often
not suitable for use with wood fuel because they can become clogged. The chimney itself and the stovepipe,
when properly installed, are intended to withstand an occasional chimney fire without danger of ignition of their
surroundings. During a chimney fire, one ought to check the roof and surroundings, and possibly wet down critical
areas. If the chimney may not be up to safety standards, one should also keep a close watch on all surfaces near
the chimney.
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