What temp starts a chimney fire?

[mnowaczyk]

I know you can't measure flue temp with a stove thermometer, but I fugure the stove will usually be hotter. It might be nice to know when I'm getting to the temps where one could occur with a dirty chimney.

I've been burning less than three days with the insert this year (and maybe 5 in the standard fireplace this fall). The chimney was super clean this spring. So I'm not too worried about the 850-900 I just hit with the stove. (I did not know my stove was even capable of hitting those temps until I put the overn thermometer on the glass.) I believe that maybe I should be concerned in the future though.

So... at what temp have you caused a chimney fire? Or known a chimney fire to occur?

 

['bert]

I am not sure I am understanding your question correctly, so if what I put here does not answer your concerns then the advice is worth what you paid for it, but here goes:

Hot fires do not cause chimney fires. Cold smoldering fires that do not get your chimney hot enough to remove the water vapor and other byproducts of combustion quickly enough cause the formation of creosote. This is also caused by wet wood etc (read around here & you will get what I am talking about) Over time this creosote builds up in your chimney & gets larger and larger. Then the one day you decide to build a hot fire (on purpose or accidentally) the built up creosote ignites and you have a chimney fire.

I do not know at what temp creosote ignites, but if it's there in an amount enough to concern you, then I would get ride of it.

If you are asking how much heat your chimney can withstand before it fails, that would depend entirely on your specific chimney. If that is what you want to know, please make sure your smoke detectors are working and your ins. is paid up.

'bert

 

[Highbeam]

I have a real probe meter for the flue and a surface meter for the stove. Except in very rare cases, the flue temp is always much much higher than the stove temp. I can easily run my flue past 1000 degrees within 15 minutes of starting the fire when the stove temp is still only a couple hundred degrees. Maybe the stone is absorbing the extra heat, I don't know, but I would never assume that the flue temp is always lower than the stove temp.

Class A steel pipe is rated for 1000 degrees continuous and up to 1800 peak IIRC. I try and bump 1000 and cruise near 900 while heating up the stove and then cruise above 700 for the rest of the burn. A lot of heat goes up the flue in a non-cat stove when you burn clean.

As far as what temp starts a chimney fire... well, that depends on what's in your chimney. You can't start one in a clean chimney since the chimney isn't burning it's the creosote. If you regularly run the flue gas up to high temps and also keep it swept then I believe that you minimize the chances of a chimney fire to near zero so long as you don't exceed the pipe's limits.

 

[leaf4952]

I keep it below 600 degrees at the bottom. Good lord ! I can't specifically speak for your stove but that sounds high in tempetature. I would consult other owners of that particular model stove configured in a similar way as you and compare notes with them. You can get good information here but you must realize you will ultimatly be the intuitive controler of your stove.
You must spend the time beside your stove, getting to know it's motions & tendancies. You can do it even working 50 hrs per week with a little help from the family after you feel confident enough to instruct them.
You must get to know your stove as well as the dryness of the wood yur sticking in . It's hard at first -- you get overwhelmed with not knowing what the (bleep) you're doing, you're hit with alot of stuff but you really need to be quiet and alone with your stove for at least 30 min undisturbed when you come home at dinner time. If ya got a kid somewhat interested - its a perfect bonding oportunity. You can make it up to the wife later if she's worth her salt.

 

[quads]

With a cheap Rutland Burn Indicator thermometer on single-wall pipe, the creosote starts to burn at about 450.

 

[Battenkiller]

I don't know specifically what temp it takes to spark off creosote in a chimney, but even if I did, that wouldn't help you anyway.

Creosote is always forming during a burn. The stuff that condenses in or near the stove at the end of a long burn gets burnt off every time you have a good fire. 600ºF or so surface temp seems reasonable since that's about how high my griddle top gets when I open it up and first see the lacy pattern of burning creosote spread across the bottom of the griddle. That's the soft velvety stuff that burns slow and brushes away easily. The dangerous stuff builds up higher in the flue.

How high? Who knows... it's where you find it if you are producing it. It depends on too many variables to predict. It's big and chunky and flaky, and if you're getting it in any quantity, it's not a good thing. It will form higher than the normal flue temps can burn it off, and therefore, it will continue to build. The higher up it occurs, the less likely it will ignite from a hot fire and the more likely it will continue to build. Since the flue gases are constantly cooling as they rise, it will take a much hotter fire to spark it off when it is way up high. But since there is a lot of it up there, once it goes it won't just burn off, it will torch your chimney.

So flue gases that exceed 1000ºF 18" above the collar (actual gas temps, read with a probe) might not have any effect on creosote that's 6' up the chimney where the gases are much cooler, but get that same 18" reading up to 1800ºF just one time and the stuff that's 6' up the chimney (and of course, everything above it) may go off in a flash. It really depends on how high up the formation is, and how much of it there is, not on the temp at the probe location.

 

[mnowaczyk]

Thanks for all the info everyone. That creosote burning number was was I was searching for. The reply about the different types of creosote at different heights is some pretty god info too. Now... I just wonder how often I should have my chimney cleaned. Assuming the stove gets cold every day... how many days of burning before I should have the chimney cleaned?

Oh... I'm probably one of the worst creosote creators... an 80's style stove with a 13" x 13" clay lined 40' chimney.

(There's a slate roof guy in my neighborhood almost every day, and he'll probably do cleanings for cheap while helping maintain other exterior things on the house... so frequest top-down cleanings should not be too hard for me.)

 

[Battenkiller]

how many days of burning before I should have the chimney cleaned?

Oh... I'm probably one of the worst creosote creators... an 80's style stove with a 13" x 13" clay lined 40' chimney.

That flue is too large and is not safe. You need to install an 8" liner for that VC. I believe 8" x 12" is the largest you can use on that stove.

 

[mnowaczyk]

I agree on the chimney liner thing. I'm still learning while keeping down my initial costs. I'm going to plan on having the chimney cleaned after about 10-15 insert burning days. I'm only 3 days in right now.

I'm pretty sure a huge problem I have right now is a cord of green wood. It's time to go buy some more wood racks and try to find drier wood. I'm shutting down for a few days while searching for good wood. I've got a few trash cans of lumber scraps I've been tossing in to try and keep temps up with this wood, but that's just creating wild swings in temperature. That's one reason for starting this post. Now I see that I'd be best to keep my stove temps under 700.

Because this all is so time-consuming, I'm basically shutting down my burning for a while. I want to:
1) search for drier woods
2) Get a good stove thermometer. Any recommendations? It seems I was getting best (hottest) readings on the glass. Can I put a magnetic thermometer on the glass with a block of metal on the back? There's not a lot of firebox metal exposed on the front of my stove. So my opinion is that the glass would provide best readings.
3) Maybe get a moisture meter for my wood. It looks like you need to split each piece to get an accurate reading though, huh? Maybe this is not worth the trouble.

Oh... the liner thing: I've researched this quite a bit. My stove recommends/requires an 8" liner. Since I don't want to destroy my regular fireplace damper (because I'd like to be able to switch back to an open fireplace)... I'd need to go through the damper with an 8" oval (I think this was 5"x10-11" last time I checked) which barely squeezes through my ~5" damper. An 8" round definitely won't get through. Also, the 8" oval is too wide to get all the way up to the straight 13"x13" clay liner. The sides hit brick on the way out of the section that connects the firebox to the straight liner. This section goes up at a slight angle and gets narrower at the top. About 4 feet of 8" oval, and it's touching brick. All of this is coming up from the bottom of course. I should be able to drop an 8" round (or oval) all the way from the top down the clay liner, but only an 8" round would fit all the way down to the fireplace damper. The problem would be getting the 8" round-to-oval converter. I won't be able to get the adapter down from the top because the 10-11" width of oval will be too wide to get through that section at the top of the narrowing smoke chamber. I also won't be able to get it up from the bottom because the damper is only 5". I assume my best bet would be to cut the adapter to a minumum possible size and bend it through, and then try to re-bend it back into shape. Maybe I could cut the adapter, and try to put it back together? Or just have a special adapter fabricated? Or maybe use 6-7 inch liner instead... maybe just for that bottom portion?

I'd hoped to get all this done before it got cold... but there are lots of things I wanted to get done that I have not. I'm happy to have an insert that burns well with the little time and money I've invested.

I agree... I don't want to burn my house down... So I'm not going to burn too much without a chimney inspection soon. I'm burning and learning.

One thing I wish I could do with this old '80s stove that I cant: Let in enough air to keep this green wood burning hot. If I open the ash pan the turbulent air seems to get the fire going, but since there's no automatic regulator on that, I need to close the ash pan opening eventually. Then there's not enough air through the thermomstatically controlled inlet to keep the fire hot. So I'm out of business until I get drier wood. Too bad I put last year's wood on the bottom of the pile somewhere. Oops!

 

['bert]

you are starting to realize where your issues are and what the steps are to correct them. Your off to a good start! IMO you can not be to careful when it comes to having a fire burning inside your house, whether it be in a stove, FP, furnace or insert. It has to be done correctly or there is a good chance of a bad / terrible result. This website is a fantastic resource and I (for one) am thankful I found it and ever grateful to those that started it and support it.

Spend some time here reading and researching and I think you will save yourself some $$$ when you decide to correct your setup and get back to burning. Best of luck to you.

'bert.

 

[carlo]

Just curious ...... before a chimney fire happens will there be embers flying out the chimney ? I know nothing is definite, but would that be a logical presumption ?

 

[quads]

Just curious ...... before a chimney fire happens will there be embers flying out the chimney ? I know nothing is definite, but would that be a logical presumption ?

No. When the embers are flying, the fire is already underway.

 

[carlo]

Just curious ...... before a chimney fire happens will there be embers flying out the chimney ? I know nothing is definite, but would that be a logical presumption ?

No. When the embers are flying, the fire is already underway.

Just in case ..... if I ever see embers coming out my chimney what should I do after calling the fire dept ? Are burning embers coming out the chimney the only way a creosite fire shows itself ? Can the fire back up into the box ?

 

[quads]

Just curious ...... before a chimney fire happens will there be embers flying out the chimney ? I know nothing is definite, but would that be a logical presumption ?

No. When the embers are flying, the fire is already underway.

Just in case ..... if I ever see embers coming out my chimney what should I do after calling the fire dept ? Are burning embers coming out the chimney the only way a creosite fire shows itself ? Can the fire back up into the box ?

Sometimes chimney fires are big and roaring, but not always. The chimney fire won't back up into the stove, but the extra draft it creates burns the wood that is in the stove very fast and very hot. If you see some hot chunks shooting from your chimney, and everything else seems ok, keep the stove shut down, don't open the stove doors, and the main thing would be to watch those hot chunks and make sure they don't start the roof on fire and/or the bushes next to the house etc.

 

[craigs]

Just curious ...... before a chimney fire happens will there be embers flying out the chimney ? I know nothing is definite, but would that be a logical presumption ?

Wow, I must have blocked out that memory, but all of a sudden I recall standing in the driveway after dad told me to watch and tell him if the roof started smoking anywhere. He was doing everything possible to keep the stove calm while the chimney burned itself out. This was with our original single wall chimney sitting atop the raging Blaze King. Even the veterans let it get out of control from time to time I suppose.

Craig

 

[mnowaczyk]

I guess 1200 degrees can be triggered in a small area with just a single ember. Then with creosote igniting, things just get hotter. Then there you have a chimney fire. eh?

I was a little shocked that a chimney sweep that recently visited told me that inserts (like mine) might create a lot of creosote, but will rarely start a chimney fire. That's because most of the heat is staying in the insert and firebox, and the draft cools quickly in my large chimney. (I need a SS liner inside the large 13x13 clay lined chimney.) So I might build a lot of creosote, but don't have to worry much about chimney fires with this setup. He said that an open fire with flames running up the chimney would be much more likely to start a chimney fire. That all makes sense, especially if the time that I'm burning hot in my stove... I'm burning clean... with no smoke or embers flying up the flue.

I've learned a lot in the last year. Thank you hearth.com members

 

[quads]

I guess 1200 degrees can be triggered in a small area with just a single ember. Then with creosote igniting, things just get hotter. Then there you have a chimney fire. eh?

I was a little shocked that a chimney sweep that recently visited told me that inserts (like mine) might create a lot of creosote, but will rarely start a chimney fire. That’s because most of the heat is staying in the insert and firebox, and the draft cools quickly in my large chimney. (I need a SS liner inside the large 13x13 clay lined chimney.) So I might build a lot of creosote, but don’t have to worry much about chimney fires with this setup. He said that an open fire with flames running up the chimney would be much more likely to start a chimney fire. That all makes sense, especially if the time that I’m burning hot in my stove… I’m burning clean… with no smoke or embers flying up the flue.

I’ve learned a lot in the last year. Thank you hearth.com members

You have to worry even more about a chimney fire than other people! If you build up a lot of creosote with your setup, and none of it burns out, yours is the kind that when it finally does burn, and it will someday burn, you will have a large and destructive chimney fire.

 

[mnowaczyk]

You have to worry even more about a chimney fire than other people! If you build up a lot of creosote with your setup, and none of it burns out, yours is the kind that when it finally does burn, and it will someday burn, you will have a large and destructive chimney fire.[/quote]

Agreed. You bet. That's why I've had a sweep come out and inspect / sweep for about every 3/4 cord that I've burnt. They keep laughing at my fear of a chimney fire, and walking away saying there's not much to clean.

My goal is to clean the creosote out instead of burning it out. Will I still have a chimney fire that way?

 

[mnowaczyk]

i'd ask the sweep where how in the chimney the creosote had deposited, then i'd take the creosote & burn it if only to get an idea of how much energy was there for a potential chimney fire.

So take all the creosote that ends up in the shop vac, and then put it back in the stove with some paper?

Wouln't it be funny if that all just went right back up the chimney and got stuck to the cold chimney walls!

 

[mellow]

Not just embers, but thick dark stinky smoke will be spewing out if your having a chimney fire. As the fire burns up you will see it coming out of the chimney.

 

[quads]

You have to worry even more about a chimney fire than other people! If you build up a lot of creosote with your setup, and none of it burns out, yours is the kind that when it finally does burn, and it will someday burn, you will have a large and destructive chimney fire.

Agreed. You bet. That's why I've had a sweep come out and inspect / sweep for about every 3/4 cord that I've burnt. They keep laughing at my fear of a chimney fire, and walking away saying there's not much to clean.

My goal is to clean the creosote out instead of burning it out. Will I still have a chimney fire that way?

If you keep the creosote reasonably cleaned out, you won't have a chimney fire. You are right, that's the best way, aside from preventing creosote in the first place.

 

[chrisman34]

Best way to put out a chimney fire if one starts IMHO......short blasts from an abc extinguisher. open door and damper then a quick blast...maybe a second ...and shut everything down for about 30 seconds......repeat Do not do one long blast as that will not allow the chimney to cool enough and will restart, and you have no extinguisher left! This worked well at my house, and at others. I am on the fire department and this is our primary attack unless it has already broke out into the structure. Once the fire is out (for the most part) we sweep the chimney(with a chain or metal brush) to get the rest of the smoldering stuff out.....then a scan of the house, both visual and with a thermal camera to make sure there is no extension. Next we "red tag" the system and call code enforcement. You can not legally burn until you have a sweep come and inspect it.

 

[Battenkiller]

You have to worry even more about a chimney fire than other people! If you build up a lot of creosote with your setup, and none of it burns out, yours is the kind that when it finally does burn, and it will someday burn, you will have a large and destructive chimney fire.

Damn right, quads! And I like the way you added, "it will someday burn".

In addition to ignition, creosote needs oxygen to burn - just like every other fuel. In a properly burning stable fire vented into the correct size flue, oxygen isn't exactly in abundance of the far side of the burn zone. Most of it gets used up in the fire itself. That's why it is usually introduced later along the way to initiate secondary combustion. But take a situation where you have a lot of extra air in the oversize flue itself plus an excessive amount of creosote, open up the air intake (adding even more excess air) to bring the flue temps up to 1000-1200ºF and you have the recipe for the perfect firestorm.

I was talking to a stove tech in my town last week about creosote and chimney fires. He said he used to give safety demos to the local FDs, where he would get a big gooey blob of fresh creosote and try to light it with a Bernzomatic torch. He said the stuff would just sit there and bubble and pulse on the top of a cement block, but he could not get it to ignite. Again, the torch is using up the oxygen and robbing the potent fuel of the other part of the chemical equation. "Creosote's funny that way", he said. "In one flue, it can sit there in a chimney until the whole flue is blocked, but never be in any danger of starting a fire because nobody ever burns a hot enough fire to ignite it. Yet get that same amount of creosote in another flue and get it up to temp (he said about 1000º) with enough extra air, and the whole of the accumulation goes off at once."

 

[Chettt]

My question is how likely is the creosote that is further away from the stove to ignite spontaneously if there is no chimney fire starting in the section of flue closest to the stove? It seems to my thinking that a chimney fire must start close to the heat source and then continue burning further up the flue. When it warms up for a day in January I pull the lower section of pipe that connects to the stove and brush out all the creosote figuring that my once per year complete clean in the summer will be fine if the first 10 feet are cleaned mid season. Am I wrong with this logic?

 

[chrisman34]

What about flying embers....leaves, paper, other sparks? If you burn right, once a year should be fine. If not I wouldn't count on 10 feet being good enough....if you have it apart anyways do the whole thing, it only takes another 5 minutes to do the whole thing.....right?