# At What Temperature Does Wood Combust ?



## Ductape (Dec 4, 2012)

The discussion here about chimney fires, and some recent news events got me wondering.

Mostly I'm wondering about wood construction materials..... kiln dried framing, roof sheathing and such. Curious how hot a chimney needs to get before roof sheathing starts to burn. 

Also open to discussion due to recent fires reported in the news. Kindling material such as newsprint, cardboard etc, left to close to the wood stove. How close can you have a little wood stacked near the stove for use later in the day?


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## chucker (Dec 4, 2012)

when i was younger and some of my time was spent with the oregon state forest service, it seemed to be 750 degrees ifin im remembering right for self/ combustion.... im sure some one will be along to correct my memory shortly???


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## winland (Dec 4, 2012)

There was an older book by the title of Fahrenheit 451. 

I pulled this info from some web site.


> auto-ignition point of paper, from the high 440s to the low 450s, but more recent experiments suggest it’s about 30 degrees hotter than that. By comparison, the auto-ignition temperature of gasoline is 536 degrees, and the temperature for charcoal is 660 degrees.
> It would take a few minutes for a sheet of paper to burst into flames upon being placed in a 480-degree oven, and much longer than that for a thick book.



Chuck


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## tbow388 (Dec 4, 2012)

*My 2 cents*

It combust right after the point right before it combusts.

Thats my best answer!:yoyo::yoyo:


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## timbrjackrussel (Dec 4, 2012)

Check out Pyrolysis - Wikipedia, the free encyclopedia


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## Mac88 (Dec 4, 2012)

Ductape said:


> How close can you have a little wood stacked near the stove for use later in the day?



Our kindlin' box is less than a foot behind our Vermont Castings. It gets slightly warm if we have a hot burn going. I think the issue with roofing is how much air space you have. If you have a stack running through an upper level floor you always use a firestop, which stands the pipe away from the structure. The air space reduces the heat transfer rather quickly.


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## brenndatomu (Dec 4, 2012)

This is a interesting topic. My Yukon wood furnace owners manual lists wood as 435*F ignition. I'm sure that varies with moisture content and wood species. 

When I was installing the Yukon this summer, some of the clearances they wanted on the duct work seemed curious to me. I guess I understand the 6" clearance for the supply plenum and the first 6' of run. But why is 1" OK after that 1st 6' (seems like a dramatic, rapid change) or after the 1st 90* turn. Why does a 90 change the clearance requirements? It would seem to me that as long as the duct work is not touching the floor joist, you'd almost have to have a fire IN the duct for a period of time to light any adjacent wood. In my research the only thing I could come up with is that, over time, the elevated temps next to the ducts actually cause a ultra slow pyrolysis to occur. Eventually lowering the ignition temp of your house framing, etc. Dunno? If someone on here is knowledgeable 'bout these things, it'd be interesting to hear some in-the-know factoids.

Hope this all plays into your original question Ductape.


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## timbrjackrussel (Dec 4, 2012)

brenndatomu said:


> This is a interesting topic. My Yukon wood furnace owners manual lists wood as 435*F ignition. I'm sure that varies with moisture content and wood species.
> 
> When I was installing the Yukon this summer, some of the clearances they wanted on the duct work seemed curious to me. I guess I understand the 6" clearance for the supply plenum and the first 6' of run. But why is 1" OK after that 1st 6' (seems like a dramatic, rapid change) or after the 1st 90* turn. Why does a 90 change the clearance requirements? It would seem to me that as long as the duct work is not touching the floor joist, you'd almost have to have a fire IN the duct for a period of time to light any adjacent wood. In my research the only thing I could come up with is that, over time, the elevated temps next to the ducts actually cause a ultra slow pyrolysis to occur. Eventually lowering the ignition temp of your house framing, etc. Dunno? If someone on here is knowledgeable 'bout these things, it'd be interesting to hear some in-the-know factoids.
> 
> Hope this all plays into your original question Ductape.



In my research the only thing I could come up with is that, over time, the elevated temps next to the ducts actually cause a ultra slow pyrolysis to occur. Eventually lowering the ignition temp of your house framing, etc. 

Exactly


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## Marc (Dec 5, 2012)

Ductape said:


> The discussion here about chimney fires, and some recent news events got me wondering.
> 
> Mostly I'm wondering about wood construction materials..... kiln dried framing, roof sheathing and such. Curious how hot a chimney needs to get before roof sheathing starts to burn.
> 
> Also open to discussion due to recent fires reported in the news. Kindling material such as newsprint, cardboard etc, left to close to the wood stove. How close can you have a little wood stacked near the stove for use later in the day?



So what you're really looking for is the autoignition temperature of wood. My SFPE handbook is at home, but it probably lists it.

Usually in fire protection applications, we're more concerned about the required energy flux for ignition, but your concern is headed in the right direction.

Although every chimney fire I've seen with extension into the house, it hasn't been because the brick has become too hot on zero clearance areas. It's usally old chimneys with comprimised clay liners or unlined brick and some mortar breaks out or a brick cracks due to the heat and you have direct flame impingement on a combustable material.

The gradual heating would get from a hot brick touching wood would cause a carbonaceous char to form actually increasing the surfaces autoignition temperature, so it would be quite a bit higher than the average book value for hard or softwood or whatever you're interested in.


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## Marc (Dec 5, 2012)

timbrjackrussel said:


> In my research the only thing I could come up with is that, over time, the elevated temps next to the ducts actually cause a ultra slow pyrolysis to occur. Eventually lowering the ignition temp of your house framing, etc.
> 
> Exactly



Actually, the opposite is true. As you expose a complex material like wood to heat, and pyrolysis with no combustion occurs, the lighter and more volatile compounds are liberated first increasing the ignition energy required for the material left behind.


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## Marc (Dec 5, 2012)

I'm sure no one wants to hear this either but the ignition of solids is a very complex thing, which is why you'll have a harder time pinning down an autoignition temperature than for a liquid fuel source, which is a little less accurate than a gaseous fuel.

Remember that for single molecule to undergo the rapid oxidation we call combustion, the molecule has to be in the gaseous state. So really, only gas can combust, despite the language we use to refer to fuels in solid or liquid phase. The volatile gas must be liberated with a heat source from the solid or liquid. In solid fuels this is actually a chemical reaction in itself, rather than just a phase change, as molecules decomposue and liberate combustable gas.

Ignition _energy_ is much easier to measure, and frankly more practical when speaking of fire spread, which is why you'll find much more information on that if you went and looked. I mean, no matter what, this discipline is not what you'd call mainstream. It's still a comparably young science. So when speaking of iginition energy, you're typically talking about an energy flux, heat in this case, and usually radiant is the dominant mode (as opposed to conduction, convection). It's measured in the lab for a solid phase fuel sample using what's called a cone calorimeter which is just a calorimeter (measures heat output via gas analysis or some other means) coupled with a cone shaped electric heating element a known distance from your fuel. So your incident heat flux is either calculated or directly measured alongside the sample and is recorded at ignition. The temperature of a solid material is a lot more difficult to measure directly and accurately and as I alluded to before, the autoignition temperature will vary greatly not only with variations in the non homogenous solid material, but also as the material make up changes chemically via pyrolysis, pre-ignition.

We use ignition energy much more often because when you're talking about a fire burning alongside combustible solids, if you know the size of the fire you can generally calculate the incident heat flux onto adjacent combustibles more easily. Then knowing the ignition energy you can predict how fast the fire will spread and thus how fast it will grow.


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## Whitespider (Dec 5, 2012)

I'd love to have a long adult argument with *Marc*... The problem is finding a topic where I can actually stay toe-to-toe with him :msp_w00t:


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## Marc (Dec 5, 2012)

Whitespider said:


> I'd love to have a long adult argument with *Marc*... The problem is finding a topic where I can actually stay toe-to-toe with him :msp_w00t:



Thanks spidey. Remember I did sacrifice a significant amount of time and money at the alter of higher education so I could bloviate about this very topic.

And we all have our strengths. You could lay down a professional looking bead in a complete circle around me in the time it takes me to break off the rod I've welded to or burned through the work piece twice and swear at it a bunch of times.


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## deutzman (Dec 5, 2012)

I've witnessed ignition several times over the past 18 years working in an OSB mill. Dried wafers with core moisture around 6% and surface around 8% and these will vary a little in the drying process. Temp of the press runs 410 to 415 range. That's heated oil in the platens. If there's anything to stop the line that's when we have to be very careful if the press is loaded and can't unload. If it looks like we'll be down over 30 minutes we can dump the hot oil and cool the press down to like 385 and have no problem. Course we use wax and resin and that may help ignition a little. 

It's a sight to see 14- 8'x24' mats of unpressed pine wood ignite. Only takes seconds to be engulfed. Fire fills the area with smoke fast. Spray with water and have to be careful you don't get scalded by the hot water falling back on you. If you can't knock it down in seconds it's a run (feet don't let me down this time) situation and let the sprinkler heads do their job. Once the smoke gets down to your head level it's time to get out fast. So I'll say 410 for 35 min. or a little more with the right moisture content can get a good fire going. Once it starts then the smoke burns too.
RUN.:yoyo:


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## brenndatomu (Dec 5, 2012)

Well, maybe no one else wants to hear it Marc, but I do. I'm glued to this thread! (yah, I know, easily entertained, right!?) 
So what you are saying is, the 70 year old douglas fir framing in my house would take MORE (heat + time) to ignite it than when the house was built? The house originally had a coal fired forced air system. Most of the duct work is still original. I notice most of it has a little rise in the runs, I'm sure that was so the heat would keep moving during a power outage. 70 yrs. ago, power outage was probably frequent, possibly lengthy.
My neighbor, who is a old tin knocker by trade (helped me with my install) seemed to be a lot less concerned about keeping the recommended duct clearances than I was. He has had a bit of training on this subject matter, and is a pretty no nonsense type of guy. He said has has wood fired furnace ducts in his house that are a fair amount closer to framing than mine. Maybe he knows a thing or two, eh? Like he said, if there were no problems in 70 yrs, with the clearances I had, considering the temps that a coal burner can make, likely not gonna be an issue from a wood furnace now.

So what do you know about that whole clearance change after a 90* turn thing Marc?

Sorry Ductape, not tryin to take over your thread, I hope this is all still relevant to your OP. I'll start another if you'd like.


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## Ductape (Dec 5, 2012)

brenndatomu said:


> Sorry Ductape, not tryin to take over your thread, I hope this is all still relevant to your OP. I'll start another if you'd like.





Completely relevant. The first thing that got me thinking about this is the construction of our own home. 1880s New Englander with a single layer brick / mortar original chimney. After we bought the house, I had an installer put in a stainless flex liner, with a poured in vermiculite / mortar insulation around the liner. Not much chance sparks can get to my roof sheathing, but it got me wondering how hot the 1 inch pine boards would need to get before I needed to worry about combustion. I sweep my liner twice a year, but these are things i think about, both in normal operation ( I probably run my stove a little harder than some folks on here) , and in the event of a chimney fire. Gotta think a chimney fire will be over a thousand degrees in seconds ??

Also, there was a recent fatal fire, I think in Maine. News reports stated there was a box of kindling (or was it actually splits) in a box near the stove that ignited during the night. Obviosly sparks and flying embers when we load our stoves is something different, but it got me thinking that maybe there was more to the story. How close to the stove must the box of kindling been?? Defies logic anyone would leave one touching the stove....

I'm also surprised my framing / sheathing in my house is less likely to combust than new framing. Honestly, the wood sheathing around my chimney is partially blackened. I've always wondered if there have been chimney fires here long ago where the previous home owners were really, really lucky.


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## stihly dan (Dec 5, 2012)

The difference of after the 1st 90 is the time the heat takes to move around the 90. AS it is always cooling down from leaving the fire box.
I would not put to much stock in what a tin knocker says. They for the most part put the square in the square hole and the circle in the circle hole, but don't know why.
I can tell you on the combustion of sugar maple, it took 1hr 25 min for a not seasoned enough split, sitting on 2 fire bricks on top of the stove to catch fire.


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## Marc (Dec 6, 2012)

Ductape said:


> I'm also surprised my framing / sheathing in my house is less likely to combust than new framing. Honestly, the wood sheathing around my chimney is partially blackened. I've always wondered if there have been chimney fires here long ago where the previous home owners were really, really lucky.



Well I'll add the caveat that the decomposition of a cellulosic material like wood probably doesn't begin in earnest until it reaches temperatures well above your household ambient conditions. I also don't know how any breakdown of cell walls, etc. over time in seasoned wood affects combustibility. Stable, seasoned wood probably doesn't being liberating more volatiles (remember it looses some with the moisture in seasoning/kilning) until exposure to a significant amount of heat.

Funny you mentioned the blackened wood as that will take higher temperature to combust. I've got some of that in my house. My dad told me that chimney fires were pretty regular occurances, and they got into the attic on more than one occasion. Actually, two of the three chimnies in the old house are not original because two fell down from chimney fires.

And here's a good story for you about that... one chimney in the house had two thimbles on the first floor, one in the kitchen and one in the sitting room. The sitting room thimble had a plug in it because no stove was there at the time and my uncle apparently heard a fire going in the chimney. Common practice at that time was to gradually pour salt down the chimney to absorb heat and probably knock off the burning chunks of creosote. Well, he wasn't versed on the "gradual" technique and dumped the entire bucket down the chimney all at once. The salt descended with such momentum that it blew the thimble out of the chimeny and shot a good bit of smoldering creosote and salt into the sitting room where my aunt was sewing.

From what I understand, it was quite some time before she allowed him to sleep in the same bed again...


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## Marc (Dec 6, 2012)

brenndatomu said:


> Well, maybe no one else wants to hear it Marc, but I do. I'm glued to this thread! (yah, I know, easily entertained, right!?)
> So what you are saying is, the 70 year old douglas fir framing in my house would take MORE (heat + time) to ignite it than when the house was built? The house originally had a coal fired forced air system. Most of the duct work is still original. I notice most of it has a little rise in the runs, I'm sure that was so the heat would keep moving during a power outage. 70 yrs. ago, power outage was probably frequent, possibly lengthy.
> My neighbor, who is a old tin knocker by trade (helped me with my install) seemed to be a lot less concerned about keeping the recommended duct clearances than I was. He has had a bit of training on this subject matter, and is a pretty no nonsense type of guy. He said has has wood fired furnace ducts in his house that are a fair amount closer to framing than mine. Maybe he knows a thing or two, eh? Like he said, if there were no problems in 70 yrs, with the clearances I had, considering the temps that a coal burner can make, likely not gonna be an issue from a wood furnace now.
> 
> ...



Like I said to Ductape, I don't want people to be under the impression that their old timbers are significantly more fire resistant than new ones. The difference may not even be measurable. Visible charring does make a measureable difference.

Regarding your duct work...I'm afraid I can't help you make sense of the prescriptive code requirements, and though this may disappoint you, there may not be a good answer. A lot of prescriptive fire and building code do not have hard science, empirical data and research behind their reasoning. They mostly come from experience with what does and doesn't burn a building down, often with a healthy factor of safety built in. Think of them as more reactive than proactive. A well known example, we have panic hardware and outward swinging doors on public buildings because we killed 400 some odd people at the Coconut Grove night club fire in Boston with inward swinging doors.

There has been a movement toward performance based codes recently with advancements in fire protection science and modeling, but we can only go so far as we aren't likely to have every local AHJ trained and knowledgable enough to know how to evaluate designs to a performance code anytime soon. In a way, they need presciptive codes just from a practical standpoint. They can much easier see that you've used the correct number and locations for your smoke detectors than they can read and verify the results of a FDS run showing likely ignition sources, fire growth rates and smoke transport for effective occupant signaling.


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## brenndatomu (Dec 6, 2012)

OK, thanks Marc. I guess that explains why I couldn't seem to find much information on the matter.

Isn't it interesting that something like fire, that we have used for thousands of years, still isn't COMPLETELY understood in 2012?!


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## Guswah (Dec 27, 2017)

I'm building a Russian banya (sauna) in a standalone building. In a traditional banya, a woodstove projects through a passage in the wall and into a separate room where wood is fed into the stove thus separating it from the "hot" room. I framed the wall that separates the two rooms with fir lumber and left an opening with about four inches clearance all around the stove. It's a little late, but now I'm worried that the opening might have been bigger and that heat from the woodstove in the opening could eventually start the wall on fire.

Currently the entire wood-framed wall is fiberglass insulated and clad with fireproof drywall. Over that, I've installed a layer of quarter-inch cement backing board. The opening itself is likewise covered with those materials, but I've also cemented in a course of refractory bricks around the opening's perimeter. The space now remaining is about an inch and a half all around the woodstove.

I wish I knew what the shell temperature of the woodstove will be once it is operational, but I do not. It's a Fisher stove -- the old type without double walls, chosen specifically for this purpose. After all, we want that thing to radiate heat quickly. My question is what to do with the remaining inch and a half of clearance to best isolate the opening from the stove's outer surface where it passes between both rooms. Another course of brick? Fiberglass insulation? Nothing at all? What will provide the best protection from the stove's surface?

I'm grateful for any discussion anybody cares to offer!


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## brenndatomu (Dec 27, 2017)

The stove walls could see 8-900*...even more...what stove is it? Most stoves require at least 18" to any combustible...some even up to 3'...I don't think there is anything you can do with your current setup to keep the framing from burning eventually...other than tear it out and go steel framing, or all masonry.


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## Guswah (Dec 27, 2017)

The stove is a Fisher. Those are now obsolete, but when then were in production there were three models called "Papa Bear", "Momma Bear", and "Baby Bear". This one's a Momma, chosen because it's low and long.


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## Wood Doctor (Dec 27, 2017)

I have a stove thermometer on my Federal Airtight 288 that measures flue gas temperature in the secondary combustion chamber about 1" below the top surface of the stove. I have sometimes reached 1400 F, but the average peak secondary exhaust temperature on a cold day with a good draft is about 1300 F. Peak stove skin temperature, therefore, must be close to 1000 F but could be a tad higher I suppose.

The Fisher Momma Bear is a good wood stove. It's not really obsolete as you say. They just aren't made anymore.


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## Guswah (Dec 27, 2017)

It goes without saying that ANY wood stove has a clearance spec of eight or more inches. I have no illusion about that. But what I have done is protect the wood-framed wall as I described in the hope that the usual clearances are not necessary. In a perfect world the stove door would protrude and the stove walls are completely sealed so that sauna occupants have complete privacy and ambient heat stays within the chamber. To accomplish that, there must be materials that will prevent conduction from the band around the stove's girth and up through the firebrick I've installed and end up heating the framed wall. Those treatments are not prescribed in any building code because they are complete overkill if you are adhering to the large clearances that are typically specified for wood stoves. By the way, as a rural, remote, and detached building, I'm not required to follow any building standards.


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## Wood Doctor (Dec 27, 2017)

Today I watched my Federal Airtight 288 surprisingly kick out some sparks right through the small air vent valve opening on the left side, I have brick all the way around the stove and the nearest combustible is about two feet away from it. These sparks jumped out about a foot and right through an unbelievably small opening. It happens.

No problems here, but I often wonder why anyone would want to compromise recommended clearances. Stoves get hot and fires can throw sparks in every direction. Simple as that.


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## ChoppyChoppy (Dec 27, 2017)

deutzman said:


> I've witnessed ignition several times over the past 18 years working in an OSB mill. Dried wafers with core moisture around 6% and surface around 8% and these will vary a little in the drying process. Temp of the press runs 410 to 415 range. That's heated oil in the platens. If there's anything to stop the line that's when we have to be very careful if the press is loaded and can't unload. If it looks like we'll be down over 30 minutes we can dump the hot oil and cool the press down to like 385 and have no problem. Course we use wax and resin and that may help ignition a little.
> 
> It's a sight to see 14- 8'x24' mats of unpressed pine wood ignite. Only takes seconds to be engulfed. Fire fills the area with smoke fast. Spray with water and have to be careful you don't get scalded by the hot water falling back on you. If you can't knock it down in seconds it's a run (feet don't let me down this time) situation and let the sprinkler heads do their job. Once the smoke gets down to your head level it's time to get out fast. So I'll say 410 for 35 min. or a little more with the right moisture content can get a good fire going. Once it starts then the smoke burns too.
> RUN.:yoyo:



By chance do you work for JM Huber in Crystal Hill? My Dad works for them at the OSB mill in Maine. Back in the 90s we almost moved to VA for a job they offered him.


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## Guswah (Dec 28, 2017)

This is a Fisher stove. There are no side air vents. The only place for sparks to fly out is the door, and there are no clearance issues there. Regardless, my post was not about sparks. It was about heat convection from a 5" x 20" area on the stove's topside where it passes through an opening in a wood-frame wall. I'm not sure how many times I need to describe the scenario.


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## Guswah (Dec 28, 2017)

I've just found this at a website called Hearth: "The benchmark temperature, which determines safe clearances, is typically 115 degrees F over the ambient room temperature. In other words, if the ambient room temperature is 70F, the benchmark wall temperature cannot exceed 185F." I think that is what I was looking for because it allows me to monitor the susceptible area, and if there is a problem, I can probably overcome it with a fan or explore other remedies.


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