Newmac CL 86-96C Installation, Operating And Service Manual page 36

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. 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.
Some people start chimney fires fairly frequently, as a means of chimney cleaning. This deters very intense chimney fires and
the small ones which do happen are always under a watchful eye. Under some circumstances, this practice may be
reasonable, but generally it is a risky method to keep a chimney clean. There is always some danger of a house fire, but in
addition, any chimney fire is wearing on a chimney; the high temperatures increase the corrosion rate of metals and the thermal
expansion of masonry materials encourage crack formation and growth.
Chemical chimney cleaners are available. Opinions on their effectiveness vary, but apparently when used regularly, and as
directed, they work, and do not damage chimneys. The usual chimney cleaning method is the oldest human energy and some
kind of mechanical tool. A stiff wire brush, a heavy chain (perhaps in a bag) hung with a rope and worked up and down from
the top of the chimney, and very small brushes have all been used. Professional chimney sweeps are also reappearing.
Some people clean yearly, other after every few cords of wood burned, but there are so many factors influencing creosote build
up that such generalizations are not appropriate in most particular cases. In new installations, or when changes occur (such as
a different stove) the chimney should be checked frequently (after 2 weeks, then after a month, then after another 2 months,
etc.) until it is clear how frequently cleaning is usually needed.
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