Extra air can also be added to the flue gases in the stovepipe; 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 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 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 up because the construction is so massive. Any 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 overlap.) 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 pre-fabricated
chimney, or a rain cap. Special drip-proof adapters are available for connecting due to their swivel joints;
rigid and accordion-type leak proof elbows are available. Horizontal or gently sloping sections of
stovepipe should be oriented so their seams are on top. Joints between horizontal pipes and/or fittings
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.
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