The fire code is clear: "Chimney's and fireplaces must be inspected at least once a year: for soundness, freedom from creosote deposits, and shall be cleaned as necessary!" [N.F.P.A. 211 9-2] It's down-right dangerous not to have your chimney cleaned and inspected regularly! Chimney fires burn like volcanoes: EXPLOSIVELY!  It's too late to "learn" AFTER the creosote in in a chimney has caught fire. Creosote ignites with incredible force that burns at 2100 degrees Fahrenheit! Such extreme heat can ignite rafters on the other side of 5 inches of solid brick! And, the sudden pressure created by such extremely high temperatures is like a bomb! Flame and dense smoke BLAST out of the chimney like a gigantic roman candle! Such incredible pressure and extreme heat will blow out, blast through, burn down and destroy anything but the most solid structures! Modern prefabricated metal fireplaces are NO match for a chimney fire! The only true fire safety is prevention!

Masonry chimneys. When chimney fires occur in masonry chimneys, the 2100 degree temperatures, and the extreme pressure's that result, often crack the terra cotta clay flue tiles which line the chimney. Chimney fires are NO ordinary fires! They frequently melt dampers and chimney caps, and can actually "melt" mortar! Flue tiles expand until they crack when subjected to such extreme temperatures. These cracks then provide a pathway for flame under intense pressure to reach the combustible wood frame of the house. Enough heat can also be conducted through a perfectly sound chimney to ignite nearby combustibles.  Even if a first chimney fire doesn't seem to have harmed a home, the structure may be so weakened that a second fire will burn it down.
 

Pre-fabricated, factory built, metal chimneys. Whole states have passed laws which mandate that if ANY zero clearance fireplace chimney is ever even subjected to a chimney fire:  the whole chimney system MUST be replaced! The photograph on the left shows the results of a chimney fire on an expensive "Class A" chimney system ($125.00 per foot). "Class A" chimney systems are designed to withstand a 2100 degree chimney fire for 10 minutes.  Almost NONE of the prefabricated zero clearance chimney systems installed in California are "Class A" chimney systems. These prefabricated zero clearance chimney systems are commonly installed in apartment buildings, housing tracts, and newer homes. The interior surface of these light sheet metal flue's tend to stay abnormally cool: so creosote builds up inside them at an accelerated rate! But, many of them are flimsy, inherently dangerous structures. In reality they often do not withstand chimney fires.

How many more whole apartment complexes will be allowed to burn down, before it is realized that the fire code which requires annual chimney inspection and cleaning MUST be enforced! Consider the fact that the total cost of of one of these inexpensive prefabricated fireplaces is only about $125.00! Thus, the cost of the whole fireplace itself in a zero clearance system, is the same as only ONE FOOT of quality "Class A" chimney material!  (In Canada, "Class A" chimney's must be designed to withstand a 2100 degree chimney fire for 30 minutes!) While the total cost of a zero clearance chimney system in California is often UNDER five hundred dollars!

Moreover, if a standard zero-clearance chimney system IS subjected to a chimney fire, the physical composition of the chimney pipe itself may become altered to loose it's stainless steel properties! This is referred to as becoming: sensitized. The intense 2100 degree temperature of a chimney fire causes many of these chimneys to loose their stainless steel properties! So, after a chimney fire, these metal chimney's begin to rust! A fireplace with a corroding chimney eventually allows direct flame to contact the wood chase! For this reason, whole states have passed laws which mandate that if ANY zero clearance fireplace chimney is ever even subjected to a chimney fire:  the whole chimney system MUST be replaced!  Associations, apartment building managers, cities, and home owners alike must all take seriously the N.F.P.A. Fire Code regulation which mandates annual chimney inspection. The only "safe" chimney, is a clean chimney!

• A flue too large for the wood burning appliance, (e.g.. unlined insert);
• A restricted air supply;
• Unseasoned or rain-logged wood;
• Cooler-than-normal surface flue temperatures, (e.g.. metal fireplace chimney).

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 a 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.