Difference between "burning off creosote" and a chimney fire?

squngel said:

What is the difference, exactly? Is it simply a matter of degree, or is the "burning hot to burn off creosote" something that doesn't actually ignite inside the flue? Or is a bonafide "chimney fire" a fire that escapes the flue?

I searched the site for "creosote" and "chimney fire" but couldn't locate an answer. I know this is a very elementary question. I also know I have a buildup of about 1/16" of creosote in my flue, which theoretically I should be able to "burn off." Do I not run the risk of starting a chimney fire by doing this?

Once again my apologies for such an elementary question.

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Good question,I can't wait to see the answer. :shut:

Answer is, IMHO, this.....

Burning off creosote is often controlled, and usually is only (or mostly) in the black pipe - which is often the first cool surface the smoke encounters.

A chimney fire, or at least a harmful one, is when tar buildup is deep enough and long enough, and also when installation is such that too much air can get into there and feed it - thus, by nature, uncontrolled.

Note - these are not "real or accepted" definitions. Ideally, folks with newer stoves should be building up little creosote. But older models - or really poorly run newer ones - can build up dangerous amounts in a short time.

Given an old stove, the experienced operator will often burn hot on a regular basis, to make certain that excess creosote does not build up. In my experience, this is a typical "problem" situation:

1. An old insert into the fireplace, installed wrong - just slipped in, or slipped in with a short piece of pipe.
2. The user burns it slow for a couple weeks because it is warm outside (fall), or other reason.
3. it gets cold, so they "fire er up", igniting the tars
4. Chimney gets a LOT of air from the fact that the panels around the fireplace do not fit tightly, and there is no block off plate below the damper.
5. Attempt to starve the fire by closing the draft controls will fail, because of #4.

Honestly, we (at the shop) did not hear about a lot of chimney fires in our last decade there.....after a number of EPA stoves had been sold.

It means that i want to hear a good response to this question. :coolgrin:

Webmaster said:

Answer is, IMHO, this.....

Burning off creosote is often controlled, and usually is only (or mostly) in the black pipe - which is often the first cool surface the smoke encounters.

A chimney fire, or at least a harmful one, is when tar buildup is deep enough and long enough, and also when installation is such that too much air can get into there and feed it - thus, by nature, uncontrolled.

Note - these are not "real or accepted" definitions. Ideally, folks with newer stoves should be building up little creosote. But older models - or really poorly run newer ones - can build up dangerous amounts in a short time.

Given an old stove, the experienced operator will often burn hot on a regular basis, to make certain that excess creosote does not build up. In my experience, this is a typical "problem" situation:

1. An old insert into the fireplace, installed wrong - just slipped in, or slipped in with a short piece of pipe.
2. The user burns it slow for a couple weeks because it is warm outside (fall), or other reason.
3. it gets cold, so they "fire er up", igniting the tars
4. Chimney gets a LOT of air from the fact that the panels around the fireplace do not fit tightly, and there is no block off plate below the damper.
5. Attempt to starve the fire by closing the draft controls will fail, because of #4.

Honestly, we (at the shop) did not hear about a lot of chimney fires in our last decade there.....after a number of EPA stoves had been sold.

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Thanks Craig.

I mean for a fireplace insert installation, like this:
https://www.hearth.com/econtent/index.php/articles/making_a_block_off_plate/

Having a tight installation, whether insert or free stand, is (in my opinion) one of the best first defenses against chimney fires. It means higher stack temps (less dilution) generally, and also better ability to control the amount of air if a fire started.

The practice some people refer to as "burning off creosote" doesn't involve actual ignition of the creosote in the flue.

Every time you have a fire, some amount of liquid creosote condenses on the inside of your flue. The cooler the exhaust temperature, the more this condensation will occur, so the heaviest layers happen during all-night burns.

Two things cause liquid creosote to solidify: time, and temperature. If layer upon layer of wet creosote is allowed to build up over a series of long, low-temperature fires, it will eventually solidify over time, but often in the form we call glaze, which is extremely difficult to remove.

Here's where temperature comes in. If you make it a practice to have a hot morning fire after every all-night burn to send some heat up the flue, the liquid creosote that has formed overnight will dry out quickly and solidify in easy-to-remove granular form, not glaze. This practice also helps burn the overnight creosote condensation off the viewing window.

The overnight creosote layer that forms in the stovepipe tends to be thinner than in the chimney, because the pipe is closer to the fire and tends to stay hotter. The pipe also gets hotter during the morning fire, and the thinner creosote, exposed to the hotter temperatures, will often dry out so thoroughly it peels off in tiny shards like cornflakes, which fall back into the fire. This is often accompanied by a crackling sound in the pipe, and sparkles in the fire where the flakes land and ignite. This might be where the phrase "burning off the creosote" originated.

Good points, tom....

I'll add that the temperature of the pipe and chimney (surfaces) can be as important or more than the exhaust. The glaze I have found mostly on the tiles of exterior masonry chimneys!

And, wow, it is like enamel baked on.......heck, you cannot even ignite it with a blow torch! But I think what happens in some cases is that softer tars down low start on fire, and then warm upi the entire chimney to 1000 degree +, then the glazes softens and ignites.

Creosote can be really concentrated stuff. Just for fun, we should have an engineer here design a little rocket sled, powered by a jet turofan, which we can feed with some concentrated creosote! Eventually, we might get into space using the stuff.

Webmaster said:

Good points, tom....

I'll add that the temperature of the pipe and chimney (surfaces) can be as important or more than the exhaust. The glaze I have found mostly on the tiles of exterior masonry chimneys!

And, wow, it is like enamel baked on.......heck, you cannot even ignite it with a blow torch! But I think what happens in some cases is that softer tars down low start on fire, and then warm upi the entire chimney to 1000 degree +, then the glazes softens and ignites.

Creosote can be really concentrated stuff. Just for fun, we should have an engineer here design a little rocket sled, powered by a jet turofan, which we can feed with some concentrated creosote! Eventually, we might get into space using the stuff.

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Craig,please put the bottle down you are cut off. :lol:

With a real chimney fire, the top of the flue outside will look like a roman candle, with a sparks blasting out into the sky. Inside, at least at an open fireplace, you'll hear a deep roar like a jet engine as the chimney fire sucks air up the flue.

With a normal hot fire, creosote will get burned off without either of these things happening.

I have a question then about this.

I have 8 feet of double wall stove pipe then 13 feet of Double wall Class A Chimney. I have a Condar probe thermometer about 24 inches up the first section of pipe. At what temperature should I get too every morning to "burn off the creosote" before I flip over the CAT? I usually run it up to about 800 or so. Is that enough to keep the creosote down?

budman said:

Craig,please put the bottle down you are cut off. :lol:

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With a name like Budman, you can't be doing the cutting off......
:coolsmile:

cstrail said:

I have a question then about this.

I have 8 feet of double wall stove pipe then 13 feet of Double wall Class A Chimney. I have a Condar probe thermometer about 24 inches up the first section of pipe. At what temperature should I get too every morning to "burn off the creosote" before I flip over the CAT? I usually run it up to about 800 or so. Is that enough to keep the creosote down?

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Sounds good. With a stove like that and a chimney like that, chimney fires are about the least of your worries. This can be confirmed by inspecting the chimney at some time during the season, just so you become familiar. But I suppose if I had that setup, we'd be talking about some brown dust inside...and that would be all.

A good old fashioned chimney fire usually results in an un-scheduled visit by the local fire department.8-/
Burning off the creosote generally results in little flecks of soot in the snow closest to the chimney.:coolsmile:

Nice professional discussion of something six-packed over for years. Thx Tom and Craig for the explanation.
The NFPA (Nat'l Fire Protection Ass.) in Quincy, Mass promotes the fact that in times of high fossil fuel costs, chimney fire incidents rise. State Fire Marshalls say the same. Both the hearth industry, and fire protection and fire departments have done a lousy job promoting the advantages, efficient and safe burns of EPA wood stoves. Most who have heated with wood all their lives as well as the newer intelligensia looking for renewables, understand little about EPA mandated technology. And don't want to know.
Interesting that we don't hear much about chimney fires since the newer cat and non-cat stoves.
I've done the morning "open door" hot burn for too long, on too many stoves. It works then ?