As I meet with hundreds of members of the wood-burning public each month, I find that creosote is one of the most misunderstood topics relating to their stove. Many customers, especially those burning older non-catalytic stoves, assume that creosote is just a normal by-product of combustion. Most customers understand that too much creosote in their chimney is dangerous and can cause a chimney fire, but almost none of them can accurately define what creosote is or how to reduce its buildup in their flue. Here’s a simplified explanation:
The easiest way to understand creosote is to first understand the combustion process. First things first, wood doesn’t burn. Wait, what?! You read that correctly, wood, in fact, does not burn, rather it decomposes when exposed to heat, breaks the chemical bonds that make up the wood, and releases volatile organic compounds (VOC’s) that do burn. This is an important concept to understand because it leads to an understanding of fuel-air ratios that will become important in a moment.
One of the most common compounds released in this process is benzene. This compound is something that most of us have heard of. It’s commonly added to everything from paint thinner to lighter fluid and it’s highly flammable in relatively small doses. It’s Lower Explosive Limit (LEL) (or the fuel-to-air ratio as a percent where benzene could potentially combust) is 1.2% benzene to 98.8% air. As you can see, it doesn’t take very much to catch fire.
Perhaps the more important part of this equation to the discussion of creosote is benzene’s Upper Explosive Limit (UEL), or the percent concentration where the mix is too rich and it can no longer combust. In this case, it’s 7.8% benzene to 92.2% air. This isn’t a very big window, is it?
Now think of what’s happening inside of your wood stove when you load up the stove at night and shut the air down (and worse if you close a damper in the stove pipe)… The bed of hot coals and the initial fire heat up the chunks of wood and release VOC’s like benzene that rise up to the top of the stove. The limited air coming into the stove keeps just enough of the benzene burning to continue heating the wood and releasing more VOC’s, but there isn’t enough air to completely burn all of the VOC’s being produced. So what happens to all of these excess VOC’s? Some of them condense inside of the stove. This is apparent if you have glass doors because the glass turns black with creosote. More of the gasses are carried past the damper into the stove pipe and into the flue where the exhaust cools down and those VOC’s condense on the walls. This happens all night long, building up layer after layer of creosote in your chimney. Eventually, this restricts the flue to the point that smoke starts backing up into the house and leaking out of the joints in the stove, especially noticeable when you are starting a new fire. This is usually the point that I get a frantic phone call to come clean the chimney.
Ok, let’s recap. 1) Wood doesn’t burn, it decomposes. 2) VOC’s are released from the wood during this process. 3) VOC’s need to mix with lots of air to fully combust. 4) Unburned VOC’s condense as creosote. 5) Creosote = potential heat lost and wood wasted from not giving your stove enough air when burning.
So now we know how creosote ends up in the chimney, but then why doesn’t it just catch fire and burn away? At the temperatures that creosote is deposited (the dew point of creosote is approximately 283°F, but that number varies widely based on the density and composition of the smoke), the VOC’s are changing state from a gas to a liquid, and finally to a solid. It would require very high heat (somewhere around 1800°F for a at least a short amount of time) to raise the creosote back from a solid to a liquid and then to a gas so that it could combust. Also critically important is oxygen. If there isnâ€™t enough air escaping the stove or entering the flue from cracks, leaky cleanout doors, etc., then the volatile gasses released from the creosote as itâ€™s heated wonâ€™t be able to ignite. However, if there is enough air entering the flue to mix with the hot smoke, then the conditions are set for a chimney fire. Consider this exampleâ€¦
Let’s say that it’s mid to late December in the Northeast and you’re in the middle of a week-long warm spell. It’s not warm enough to not need the stove, but it’s too warm to run the stove with proper air without having to open all the doors and windows in the house. So all this week you’ve been smoldering your fire. The end of the week arrives with a cold snap and what do you do? Suddenly you start burning the stove with more air, and let’s say that one morning you’re stoking the new fire and you get distracted for a few minutes before you shut the air down. From the other end of the house, you smell something “hot” and briskly walk to the stove to find the pin of the stove thermometer pegged at 900°F and it sounds like someone is firing a flamethrower into your chimney. You run outside and find black “stuff” all over the roof and ground and flames are blowing out of the chimney. As you’re dialing 911, you realize that you are having a chimney fire.
So what happened here? I said the creosote needed around 1800°F to catch on fire (creosote is a fickle creature and can actually ignite at significantly lower temperatures under the right conditions, but to keep this simple…), but your thermometer only read 900°F. How could this be? First off, the thermometer wouldn’t read past 900°F, but the fire very likely was more than that. Second, your surface-mounted thermometer reads approximates half of the actual temperature of the gasses inside of the flue (most manufacturers recommend mounting the thermometer 8″ above the stove outlet on the stovepipe). As you can see, the exhaust gasses entering the creosote-laden flue were well above the required 1800°F and easily ignited the creosote, starting the chimney fire.
When you call in a CSIA certified chimney technician to inspect your chimney for damage, he finds cracks in the clay flue tiles that line your chimney and recommends that you have your chimney relined for safety. What happened was that the chimney fire got so hot, so quickly that it “thermally shocked” the clay liner—it expanded faster than the tile could respond, and it cracked the liner. The next question is always, “Is it safe for me to start a fire?” The answer, of course, is NO. The second question comes, “What if it’s just a small fire?” The answer is still no.
Is it really dangerous to burn a fire after a chimney fire? After all, the fire department came out and pulled brushes through the chimney and made sure the flue was open and scanned the walls with a thermal imager to make sure the fire didn’t spread into the structure of the house, so it sounds like everything is reset. Shouldn’t it be safe to burn again? That technician wasn’t lying to you. It really isn’t safe to use and here’s why: Those cracks the technician saw open up as the tile heats up, so what looked like a hairline fracture in the picture could easily open up to a quarter of an inch or more with a fire going. While the gap is opened up, creosote-laden smoke is settling into the gap and condensing in the void between the outside of the tile and the inside of the chimney (a place where chimney sweeps can’t sweep). The next chimney fire will likely ignite this creosote and can easily spread into the structure of your house (This is also one of the big reasons that unlined chimneys don’t meet code). This isn’t a place where gambling is ever going to pay off. Your chimney sweep should be able to give you a written quote to safely repair your flue, or put you in contact with a professional who can.
For a much more in-depth, scientific examination of chimney fires, I recommend reading Chimney Fires: Causes, Effects, & Evaluations produced by the Chimney Safety Institute of America which can be downloaded (at the time of publication) at: http://www.ncsg.org/external/wcpages/wcwebcontent/webcontentpage.aspx?contentid=588
The author is a CSIA certified chimney technician