Creosote is the residue that forms in the chimney flue when burning any solid fuel (like wood). Creosote is unburned “hydrocarbons” that collect and accumulate on the flue walls. Believe it or not, smoke is comprised (mostly) of water vapor! (Think of smoke as a “tar fog!”)
There are three forms or “stages” (or degrees) of creosote. All three stages are combustible and should be addressed.
First Degree Creosote Buildup
First degree creosote can (usually) be removed effectively with a standard chimney brush and a little elbow grease. This type of creosote is “wispy” or more like a light coating of ash. First degree creosote develops when there is good combustion of the wood and/or relatively high flue gas temperatures. Remember! Burning any solid fuel (a.e. wood/coal) is always going to produce creosote! A fire will never consume anything 100%. Never! This basically describes an open fireplace scenario. The fire has access to lots of air for the combustion process and the flue is “warm” so the smoke “flies” up the chimney.
Second Degree Creosote Buildup
Second degree creosote is a bit trickier. This type of creosote buildup is generally in the form of shiny black flakes (but not always!). Imagine dry, hard tar like corn flakes. It’s not as easy to brush away, but still removable. It would be difficult to describe all the situations where 2nd degree creosote develops, but suffice to say it will occur most often when the incoming air is restricted or when using unseasoned or “green” wood (or both). This describes “airtight” wood stoves and fireplaces with glass doors that are shut while in use. (Just an aside: Do not ever close the glass doors while having a fire! A fireplace is not a “stove” and is not designed to act as one. A fireplace needs the entire front or “face” to be open in order to operate correctly) A little more extreme method of creosote removal is in order here. This type of creosote may require that the system be “Ro-Kleaned”™ or sweeping the flue with rotating wire whips and chains! (Yes! Very aggressive!) You will notice that I do not use the term “cleaned”. A chimney system can only ever be “swept”. Once a chimney is put into use, it is never, ever going to be clean again! It is “contaminated” with creosote from the very first fire and no amount of sweeping is going to change that fact. One can only remove as much of the highly flammable creosote as possible. So be aware of a chimney sweep company that touts that they are going to “clean” your chimney. There is no such animal!
Third Degree Creosote Buildup
Third degree creosote buildup is the worst of them all. This occurs when the flue stack temperatures are low (cold chimney) and/or combustion is incomplete (think green or “unseasoned” fuel being used in an “airtight” wood stove). This is common when any of, or a combination of, these conditions exist:
- On wood stoves with the air controls turned way down
- Chimneys that are not insulated (or other reasons the chimney is cold)
- When using “green” or unseasoned wood
- If the flue is over sized (bigger does not mean better) for the appliance (stove OR fireplace!)
Third degree creosote looks like a tar “coating” on the inside of the chimney flue. Think of this as a thick black “molasses” type substance. It is extremely concentrated and is the “fuel” of a chimney fires! This coating of creosote can be compared to a very thick “paint” that will dry and then be “re-coated” with every new fire built in the appliance. When I say “appliance” I mean whatever the fire is built in. This could be an open fireplace or an “airtight” wood stove.
(Or an airtight wood stove IN a fireplace!)
Sometimes using the “Ro-Kleen” sweeping method may work to remove most of the flammable creosote, sometimes times it does not and more severe options may have to be undertaken to make the system as “safe as possible”,(remember, it is never going to be safe) to include chemical “treatments” and even the removal and re-installation of a new chimney flue (or “liner”). We hope this scenario does not happen to you, but we’ll cross that bridge when we come to it!