The Cause of Creosote

While most people think that simple “smoke” results from an initial fire in a fireplace, sometimes it’s more than that.  What’s called “flue gas” can result from the primary combustion, and it consists of steam, vaporized, unburned carbon-based by-products known as vaporized creosote.  As this “flue gas” travels upward through a chimney, it goes through the relatively cool flue where condensation takes place. Like warm breath on a cold mirror, the cool surface temperature of the flue causes the carbon particles in the warm vapor to solidify.  This solid material is called creosote.  It’s usually black in appearance, and exists in 3 different stages.  All stages are highly flammable, and all stages can exist in your chimney.

The 3 Stages of Creosote

1st stage - fine black powder 2nd stage - drippy tar / crunchy chunks 3rd stage - shiny glazed hard material

A Large Flue

When an older fireplace insert is vented into a masonry chimney, often times the flue is too large. This increases the smoke’s “residence time” and decreases draw.
  • modern specifications call for a 6″ round flue.
  • older inserts may be vented into a 13″ x 17″ flue.
In these cases, the flue rapidly builds up creosote, as the large air space cannot get hot enough.  This causes rapid creosote condensation.

Restricted Air Supply

Simply put, the longer the smoke is in the flue, the more likely 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, creosote will build up rapidly.

Wet or Unseasoned Wood

As rule of thumb, never use wet wood to fuel your fire, it creates a lot less heat, and it doesn’t do a very good job of burning the available fuel in the wood.  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.  Dry, well seasoned wood is the only wood that should be used in any chimney system.  Tar-like creosote found in an open chimney system is more often than not ,caused by burning wet or unseasoned wood.

Cooler Flue Temperatures

Believe it or not, the main cause of creosote in your chimney is a cool flue temperature.  Metal, prefabricated zero clearance fireplace chimney’s are extremely prone to creosote build up.   Their light sheet metal construction actually causes the interior flue temperature to be abnormally cool.  Due to this, creosote condenses inside a prefabricated chimney at rapid rate.

Getting Rid of Creosote

Around 25,000 fires a year occur in homes with chimney problems.  Much of this has to do with creosote buildup.  These fires can easily be prevented with a simple chimney cleaning. If you’re concerned about the creosote in your chimney, please feel free to contact Atlanta Air Duct Cleaning, your premier choice for Atlanta chimney cleaning!
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