Just exactly what is creosote?

Q: Am contemplating getting a wood stove to reduce my heating bills (I currently have an oil furnace, if you get my drift). I have enjoyed the information found on your website, especially your informative Sweep's Library, where I learned I should avoid the dreaded Creosote or pay the price in the form of even-more-dreaded Chimney Fires.

But something puzzles me. It seems to me that the only solid wood fire leftover that would be lightweight enough to be carried up the chimney by the rising hot air would be ashes, which are non-combustible. I understand that there is also water vapor present as a result of the wood combustion, and that it tends to blend with the flyash and condense on the inner walls of the chimney, so the blend paints the inside of the flue. If I'm getting this correctly, as these layers build up and dry out they somehow transform into the dreaded C-word.

What I don't get is, what makes the combination of these two non-combustible materials, steam and flyash, suddenly become combustible? You hear hair-curling stories of 2,000-degree chimney fires that spit balls of flaming goo (your phrase) onto nearby roofs until the whole neighborhood goes up in flames! How can flyash and water possibly catch fire, let alone fuel such a conflaguration?

Flummoxed Frank

Hi Frank,
In a laboratory, using an oxygen bomb calorimeter to burn bone-dry wood ( 0% moisture content ) in an atmosphere of pure oxygen, flyash and steam might be all you'd find in your chimney. In the real world, however, wood combustion is never that complete.

Incomplete combustion adds a trio of extremely combustible components to wood exhaust; namely, unburned volatiles, carbon, and wood particles, which permeate the blend that eventually condenses in the chimney.

Even the most efficient fireboxes produce SOME unburned exhaust gases, volatiles and particulates. Cook up some wine and collect the condensation, and you'll get high alcohol content brandy. Cook up some wood, and the condensation that collects in the chimney will be highly concentrated, ultra-combustible creosote.

So, you might ask, if these components of wood exhaust are so highly combustible, what keeps them from burning up in the fire?

Actually, there are several factors that contribute to incomplete combustion (and creosote formation):

Primitive stove design: Until the relatively recent development of EPA approved stoves, there simply wasn't enough heat or air in the typical firebox to enable even partial combustion of the wood exhaust. These older stoves, which we call "Smoke Dragons", sent nearly all of the unburned volatiles and particulates from the fire right up the chimney. EPA stoves incorporate a "secondary burner", where pre-heated air meets the unburned particles and exhaust gases on their way out of the stove and ignites them, burning up most of the leftovers from the primary fire.

Excess water: No matter what you do, there will always be a small amount of water vapor in wood exhaust, but every effort should be made to hold it to a bare minimum. Green or rain-wet wood can produce so much steam that it can prevent the secondary burner in even the most efficient EPA-approved woodstove from firing off. To make matters worse, this steam then mixes with the unburned exhaust, cooling it down, increasing its density and slowing its flow rate up the chimney. All of this contributes to increased condensation (creosote buildup) in the flue.

Insufficient air supply: "Airtight" stoves are called that because the operator has the ability to adjust the rate of burn by regulating the air supply to the fire. If the air control is shut down too far or too soon, there might not be enough air in the firebox to promote complete combustion, resulting in an increased amount of unburned wood gases and particulates in the exhaust.

Cold or oversize chimney flues: Since creosote condenses inside the flue as the wood exhaust cools, it is important to keep the exhaust gases hot and flowing briskly until they exit the chimney. To this end, the ideal wood stove chimney is made of insulated stainless steel, is the same size as the flue collar on the stove, and runs straight up through the room-temperature air in the middle of the house. The worst case scenario would be an oversize, rectangular masonry chimney that is exposed to cold outdoor temperatures for its entire length.

Creosote always originates as a liquid condensate, but, depending upon the variables outlined above, eventually takes one of three forms:

Stage 1: Velvet Soot. The product of hot, dry-wood fires, Stage 1 is the least combustible and easiest to sweep type of creosote.

Stage 2: Porous & Crunchy. An increase in unburned volatiles and particles in the exhaust causes this heavier, stickier, harder to sweep formation.

Stage 3: Glaze. This occurs when fresh layers of liquid creosote condense so rapidly that previous layers don't have a chance to dry out and solidify. Glaze starts out as a gooey, tarry mass, then eventually hardens like a rock, and can't be removed by conventional sweeping. To read more about glaze removal, click here.

The bottom line: Even if you're burning bone dry wood in a super-efficient EPA approved stove and always give the fire plenty of air, you should check your chimney at the end of every burning season and remove any creosote that has accumulated.