About Chimney Creosote – Part 1: The 3 Stages of Creosote

What is Creosote?

Creosote is actually just one of the components in the stuff (aside from the ash) that’s left over when wood is burned.  The whole mix of tar and creosote and soot is commonly called creosote.  The term is almost exclusively used when talking about burning wood.  If discussing soot resulting from burning oil, or even gas, this is just soot and it’s just called soot.  Though the black residue in the chimney from burning wood is called creosote, it is in fact mostly tar.

There are, generally speaking, three types of creosote are found in chimneys and they are usually called ‘stages’ or ‘degrees.’  All three forms are all combustible and should be removed.

First Degree Creosote Buildup

First degree creosote has a high percentage of soot and can be removed from a chimney effectively with a chimney brush.  First degree creosote develops when there is a relatively good combustion of the wood and/or relatively high flue gas temperatures.

This describes an open fireplace.  The burning wood had lots of air for the combustion process and the heat flies up the chimney.  These are best conditions for a chimney.

Second Degree Creosote Buildup

Second degree creosote is a bit trickier.  This creosote buildup is generally in shiny black flakes.  Imagine dry, hard tar corn flakes, and in greater volume than first degree creosote.  It’s not as easy to brush away, but still fairly removable.  It would be difficult to describe all the situations where 2nd degree creosote develops, but suffice to say it will occur where the incoming air is restricted.   This describes woodstoves and fireplaces with glass doors.

Third Degree Creosote Buildup

Third degree creosote buildup is the worst of them all.  This occurs when the flue temperatures are low and/or combustion is incomplete.  This is common when any of, or a combination of, these conditions exist:

  • On woodstoves with the air controls turned way down
  • Un-insulated chimneys (or any other reason the chimney is cold)
  • When using unseasoned wood
  • If the flue is oversized for the appliance
  • When the house is tight and can’t draw sufficient combustion air

Third degree creosote looks like tar coating or running down the inside of the chimney.  It is extremely concentrated fuel.  It can get very thick as it hardens and is recoated over and over.  An inch thick would be unusual, but it’s not unheard of.

And worse yet is third degree creosote that fills up “chimney fire fluff.”  If creosote buildup catches fire in a chimney, maybe it burns away completely but more often it does not.  More frequently the creosote partly boils, partly burns and leaves a dried out light-weight “sponge,” often more than 2” thick which is actually very easy to remove.  But if it is not removed, new third degree creosote fills that sponge you can have well in excess of 100 pounds of creosote in a chimney.

The first chimney fire may not have damaged the house, but that next chimney fire will be fiercer than the first and exceptionally dangerous.  The really tough part is that third degree creosote, in any form, is very hard to remove.

We’ll discuss ways to remove creosote in Part Two.

4 thoughts on "About Chimney Creosote – Part 1: The 3 Stages of Creosote"

Charles Hand says:

I am a home inspector. When inspecting a wood burning fireplace I wrote it up to be cleaned & Wett Certified. It was easy to see creosote, shining at the flue. A Professional Wett Inspector was previously called in to do the inspection. He said the creosote wasn’t an issue. I noticed also that he only did the inspection from the living room fireplace, never going outside to look at the chimney. The real state agent involved told me that I should have minded my business as the WETT inspector is the professional & I am not. Like most home inspectors, I studied wood burning fireplaces & have a pretty good idea what is to be inspected when a WETT certificate is required. I’ve inspected hundreds of fireplaces & am very confused with the WETT process when the inspector doesn’t pay any attention to these details.

High's Chimney Service Inc. says:

The creosote you see may or may not be a problem. A very thin sheen of creosote is pretty common and not enough to support a chimney fire. A recommendation to sweep it is reasonable of course, but might not make a real difference. It’s when the creosote gets a bit thicker, about 1/4″ they say, that it can support a fire. If the creosote was like that and not just a black sheen, you might want to push the issue.

This isn’t to say the WETT inspector didn’t miss something that mattered; I am in less a position to make a judgement than you because I didn’t see the chimney. If the inspector only inspected from the hearth, that’s below my standards anyway… and it’s below industry standard here: NFPA says Level 2 inspections for real estate transfers. (not sure if NFPA matters in Canada, but it’s good testing and standards regardless)

Interestingly, a Level 2 inspection doesn’t specifically require a roof-top inspection. It’s more or less indicated, but it’s conspicuously not mentioned. I can only assume that’s because using a ladder is outside the scope of “readily accessible” but I also think it is reasonable to expect a chimney professional to go to the top and look if at all possible. If attic, crawl space, outside of the chimney etc were “accessible” they’d have been include in a Level 2 in the US.

And I suspect the real estate person is jut mad at you because you complicated his/her life by not saying everything is fine. Inspectors ALWAYS represent the buyer, regardless of who is paying the bill. Not news to you I’m sure 🙂

Not sure this was helpful but hope it was sympathetic at least. BTW, why not get WETT certified as well?

Good luck with this!

Andy Kwit says:

We have been cleaning our chimney as long as we’ve been here Wisconsin. we have a scaffold set up for when we are cleaning it. It’s about 2 stories tall and we still put an 8′ ladder on top of the scaffold, not the safest but someone holds the ladder at the least. The point I’m trying to ask so I know it’s100& close to being done well is I have 6′ handles I put 2gether and we scrape the sides and push all the loose stuff down and I go to the basement and there is a little hatch at the bottom u open and I get out a lot of this black a little shiny flakes. Doesn’t feel like 100% tar really but all the shit we got off the inside of the chimney I picked up like 2-3 full. 5 gallon buckets of that shit that got off and after I pulled out I’m assuming was that u call creosote was that good enough4 the year or should it have been more after 1 year of every year. But I don’t know 100& if it was more or less every year we”ve done it ,the chimney from top to bottom is about30′-40 from the top to the bottom of it. I’m giving a lot of information maybe all not needed but u might get a better picture about how we do it here pretty much in the summer maybe a little later when I visit my family out in these woods. So it would be great if you have better advice to keep them safe and the chimney not catching fire. Maybe you think we should clean it better or we’re doing ok.sorry for the really long question not comment but I seen that word (creosote) on on the pipe going from their pot belly wood burning stove into the wall and the pipe is like 6′ long, and that goes into the chimney from the 1st floor. Thanks for going to read this, now hopefully I can get some worry and calm down a little bit even if the news I get is good or bad. Can u please call or text me sometime. If I don’t .. I’m probably busy at the moment and leave a phone# with a good time to get through 2 U. Thanks a lot

Andy Kwit says:

Thanks u answered so fast. Any,everything you said was what we pretty much did . Used a 2story scaffold with 8′ ladder. We cleaned it as good as we could have. Got rid of all the loose short ,went to the basement and pulled like3-5gallon buckets out of that hatch. Then I still sent my brother up top again15-20min to try to get more out of the chimney, but maybe a hand full came down. I was curious if that is sufficient enough to do once a year and is it done well enough to where that chimney shouldn’t catch on fire . Or should we bring a specialist in. I don’t think so. I read that if u don’t clean it to often that’s when that tar builds up. He said the most he’d ever seen was2 inches. He said that is very rare, so I think it should be good how we keep up with it . Thank u again!!!????????????

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