What
most people think of as "smoke" is better termed "flue gas." This "smoke",
or flue gas is released by the initial fire: the "primary combustion."
Flue gas consists of steam, and vaporized but unburned carbon based
by-products (vaporized creosote). As the flue gas exits the fireplace or
wood stove, it drafts upward into the relatively cool flue where condensation
occurs. Like hot breath on a cold mirror, the cool surface temperature
of the flue causes the carbon particles in the warm vapor to solidify.
The actual cause of creosote condensation, is the surface temperature of
the flue in which the flue gas comes in contact. This resulting carbon
based condensation which materializes inside the flue is creosote. It's
usually black in appearance. It can be the fine black dust called soot,
(1st stage creosote); or porous and crunchy, (2nd stage: see photo on left);
or it can be tar-like: drippy and sticky, until it hardens into a shiny
glaze, (3rd stage). All forms of creosote can occur in one chimney system.
Whatever form it takes, creosote is highly combustible. If it builds up
in sufficient quantities - and ignites inside the chimney flue: the result
is a volcanic chimney fire.
Several conditions encourage the buildup of creosote:
If an older fireplace insert or hearth mounted stove is vented into a masonry chimney: usually the flue is too large for the inserted stove. This increases the smokes "residence time" and decreases draw. While modern specifications call for a 6" round flue, older inserts may be vented into a 13" x 17" flue: that's 10 times too large! In such conditions, the flue rapidly builds up creosote because the large air space can not heat up enough. Not only does this cause rapid creosote condensation, it also prohibits the stove from burning efficiently! Simply relining such a chimney may increase heat out put and efficiency by 200%, and will cut creosote condensation down to minimal levels. Because the total volume of air inside a smaller flue IS less, it can stay much hotter: this causes a stronger draw. A stronger draw enables the stove to burn hotter.
In the case of older wood stoves, fully packed loads of wood (that give large but cool fires with eight or 10 hour burn times) contribute to creosote buildup.
Creosote condensation also occurs more rapidly in chimney that's on the side of the house, rather than in a chimney that runs through the center of a house.
Air supply: The longer the smoke's "residence time" in the flue, the more likely it is that creosote will form. If the air supply of a fireplace is restricted by closed glass doors, or by failure to open the damper wide enough to move heated smoke up the chimney, creosote will build up rapidly. A wood stove's air supply can be limited by closing down the stove damper or air inlets too soon and too much, or by improperly using the stovepipe damper to restrict air movement.
Unseasoned firewood. Wet wood is bad wood. Because
it is wet, it creates much less heat, and actually fails to burn up a lot
of the available fuel in the wood. Because so much energy is used initially
just to drive off the water trapped in the cells of the logs, burning green
wood causes the whole fire, and the flue to stay cool. The "smoke" of unseasoned
wood is heavily laden with unburned creosote. Because unseasoned wood causes
the whole system to burn cool, the creosote laden flue gas quickly condenses
on the surface of the flue. Only dry, well seasoned wood should be
used in any chimney system. Third stage glazed tar creosote in an open
chimney is almost always caused by burning wet, or unseasoned wood.