Creosote Buildup

Residence Time - Residence time is the time the smoke remains in the venting system. Residence time is affected mainly by flow. Flow is simply the amount of air moving through the chimney. Within certain limits, the greater the air movement, the lower the residence time will be. Abnormally high flow can result in turbulence, which can actually increase residence time. Low flow, usually caused by oversized flues, results in longer residence time and high rates of creosote buildup. The less time the smoke stays in the chimney, the less it can deposit on the walls. This is desirable for maintaining a clean chimney.

Smoke Density - Smoke density is the amount of smoke produced by the fire. When combustion air is restricted, it causes incomplete combustion resulting in high smoke density. Creosote is simply unburned particulates and gases leaving the fire. When smoke density is high, creosote deposits will be high. Without smoke there is no creosote.

If you can visualize an open campfire, it generally produces very little smoke because it has plenty of air to complete the combustion process. With complete combustion there is no smoke and no creosote.

Stack (Chimney) Temperature - Stack temperature is the temperature of the interior walls of the chimney. Stack temperature is affected by the amount of heat allowed to go up the chimney. The warmer the walls of the chimney the less the creosote can adhere to them. Stack temperature is also affected by the size and location of the chimney. If the chimney is on an exterior wall as opposed to an interior wall, it will constantly be subjected to outside cooling effects. If the chimney is oversized, it will require more heat to keep the surface walls warm due to the increased mass. With this thought in mind, you can see how a very large chimney that will never fully warm up would have the potential to cause excessive creosote deposits.

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