Chimney Fire Progression

[LeonMSPT]

"Always"? is pretty often.

What if there is no creosote down low to ignite? But the person burns a very hot fire which keeps the lower part of the chimney and the pipe burned clean? However, the top part of the chimney, sticking out into the 30 below air up high, cools enough often enough to accumulate some measure of creosote? One day, it gets "just hot enough due to the hot fire... 650 degrees or so, and it has enough air, it'll ignite.

I have fought some chimney fires over the years. Seen the chimney plugged high, burning like a torch. Go up there and drop the weight on the chain, throw in the "goody bag", and let things cool off. Sometimes you have to put two or three bags in, because it keeps flashing back into flames...

When it's cool enough, we'd clean it and check it over. Either tell them, "Go ahead and fire it back up.", or "It's condemned, don't fire it until it has some work done on it. It's not safe." Should they decide to fire it anyway, their insurance company is unlikely to cover their losses.

"Always", is the opposite of "Never"... neither really exists in our lives.

Does anyone know how a chimney fire starts?

Does it always start from the stove pipe and move upwards?

Can it start in the top of the chimney from a spark?

If you're stovepipe and most of the chimney is very clean, but some creosote has accumulated near the top roof exit or on upper elbows, can it still happen?

Thanks

I think chimney fires always start lower down and progress upwards. I don't see how it is possible for the heat and flames to pass by and not ignite the creosote build up on the lower part of the flue and ignite it further up.

This is my understanding of how a chimney fire can start and can progress under the right circumstances. Somebody correct me if I'm wrong.

Creosote accumulation it is likely to form mostly near the top, coolest part, of your chimney, but if you burn cool fires and/or damp wood you will also get accumulations in the stove pipe closer to the stove. (If you have black build up on your glass you are getting it in the lower part of your chimney).
The thicker (most dangerous) accumulation of creosote are generally out of reach of the heat that goes up the flue, but the lower part is not. If you burn hot enough you will not get any significant creosote deposits near the lower end of you chimney (stove pipe), but if you burn cool fires coupled with moist wood you can and will get deposits in the lower parts of your flue. If you get enough deposits and burn a particularly hot fire those deposits in the lower portion can, and will, begin to ignite. Depending how thick the creosote deposits are, how hot your fire is, and how long you let it burn for will determine what happens next.
If you have relatively thin deposits the creosote will likely just burn off and you won't even know it is happening.
A medium deposit may start making a lot of popping and crackling that you may hear, and it will progress up the flue (towards the thicker deposits), this is when it would be a good time to close your damper all the way as it is still basically being sustained by the fire in the stove, and likely this will be enough to starve the progression of the creosote from being burning off.
If the deposits are not that thick the last sort of "chimney fire" will often go out on it's own accord as it progresses up the chimney away from the heat of the stove, however if your creosote deposits are thick enough, and it is able to burn far enough up your chimney, and your stove fire is burning particularly hot, and the damper is wide open allowing plenty of extra oxygen to flow up the chimney it can start to become self sustaining and literally create it's own real fire and draft and you then have a full fledged chimney fire on your hands.

 

[Lumber-Jack]

"Always"? is pretty often.

What if there is no creosote down low to ignite?

Well I guess if there is no creosote down low to ignite then there will be nothing to burn down there, but I'm pretty sure it will start at the lowest (hottest) point in the flue where there is creosote build up and burn up, not down. Low and high being relative. Creosote will get deposited differently if in every different situation, not every chimney fire is going to start at the same place.
I just don't see how it would start at the upper (cooler) region of the flue and burn down, especially in a case where there are no creosote deposits down there to burn.

So yeah, I still think it will "always" ignite at the lowest place the deposits start and burn up from there.
Unless someone can explain to me how it can ignite the cool upper region and fail to ignite the lower hotter deposits.

 

[dolphins1lrb]

That's truly another avenue for the development of a chimney fire. Likely as prevalent as a connector pipe fire spreading into the chimney. Watching stack temperatures seems to be the key, if your chimney is dirty...

Just cleaned the connector pipe and the BDD I installed last week. Just a layer of dust, but wanted to check and see how things looked at a week with the BDD connected and working at the proper draft. Seems to be less junk than without it.

Certainly maintaining a lower stack temperature under extended high fire. Highest I've seen was 450 degrees, at idle it drops back to 200... to maintain a higher idle temp I'd have to give it go much air it would certainly go to 600 or more under high fire. Don't want that.

We have had two chimney fires, and both occurred with creosote accumulation at the peak of the roof, where the insulated outside chimney poked up into unrestricted air space. The Fire Chief told us that the creosote condenses at the top of the chimney, which is cooled by prevailing wind and stack cooling. A hot exhaust into this obstruction then ignites at the ignition temp.

THat's what I always thought it happened, at the end of the stack everything cooled and accumulated there and the next hot fire would set everything off whereas in contrast buildup at the bottom- just above the stove if that were to light up it would be suffocated during the run up the pipe. While I've not had a chimney fore I've been freaked enough times I've gone outside and looked at the top of the chimney for flames, being a noob................

Can you tell me what a BDD is?

 

[LeonMSPT]

BDD = Barometric Draft Damper... like on your oil burner... hot water heater.

It's a wood/coal boiler, and the manufacturer recommends no more than .04 inches of draft. Without the BDD, the chimney pulls about .15 inches of draft. Had to actually move the hose on the manometer to measure it, nearly sucked the little ball right out of the thing on the low side.

I've considered the issues with chimney fires, excessive draft, way excessive stack temperatures and flames being sucked into the chimney connector... Determined I am better off with the BDD than without it.

The chimney will have to be checked and kept clean, and I redid the chimney connector pipe with black steel "T"s and two short pieces of pipe. All I need to do to clean it is take the caps off and run a brush through a few times, take a hand brush and clean the "T" with the BDD in it. Vacuum out the dust, and put the caps back on...

Looking into overhauling the entire piping system in the spring with heavier stainless... insulated with larger heat protection overhead... flame protection in this case if there should be a chimney pipe fire when I am not home.

My risk-benefit analysis of BDD vs no BDD was simple.

All I had to do was use the galvanized pipe for a period of time.

1. Whenever the boiler was fired, high or low, the chimney was putting out nasty smoke.
2. Stack velocity was near zero in a 35 ft tall chimney, allowing things to condense out and stick rather than go on out the chimney like it's supposed to.
3. When I pulled the galvanized off, there was 1/4 inch of fly ash and creosote accumulated in it after a month of burning. It ignited when I was grinding a screw off because it was stripped out.
4. Feeling, as I do, that many chimney fires result from a connector pipe fire, I am eager to prevent one from starting if I can.
5. Stack temperatures were high enough that the zinc plating turned grey and was smelling up the basement. Indicated at least 800 degrees on the elbows where this occurred.
6. Stack temperatures below 450 degrees now, on the boiler side of the BDD.
7. Stack temperatures as much as 250 degrees above it.

Considering going over to coal exclusively in it. Wood is a faster fire with quicker heat... thinking when I get home from work and it's out or near out, relight with hardwood and get it up to temperature or mostly there. When the wood burns down to coals, begin building a coal bed... It takes time to work up a coal fire. Push too hard and you get gas accumulations and minor explosions. They're a bit unnerving, but don't seem to do a great deal of damage. Not an emergency, but better avoided if possible.

 

[LeonMSPT]

Creosote also melts when it gets hot... and chunks break off and fall back into the cleanout area.

Picture a nasty cleanout area, with burning and melted creosote running and falling back into it...

Not pretty, seen it. My advice to anyone with an active chimney fire is to not open cleanout doors unless they're standing there with a hose connected to a truck, wearing turnout gear.

"Always"? is pretty often.

What if there is no creosote down low to ignite?

Well I guess if there is no creosote down low to ignite then there will be nothing to burn down there, but I'm pretty sure it will start at the lowest (hottest) point in the flue where there is creosote build up and burn up, not down. Low and high being relative. Creosote will get deposited differently if in every different situation, not every chimney fire is going to start at the same place.
I just don't see how it would start at the upper (cooler) region of the flue and burn down, especially in a case where there are no creosote deposits down there to burn.

So yeah, I still think it will "always" ignite at the lowest place the deposits start and burn up from there.
Unless someone can explain to me how it can ignite the cool upper region and fail to ignite the lower hotter deposits.

 

[Lumber-Jack]

Creosote also melts when it gets hot... and chunks break off and fall back into the cleanout area.

Picture a nasty cleanout area, with burning and melted creosote running and falling back into it...

That makes sense, I guess that is one way the fire could "fall or drip" back below the point of ignition" :zip:

 

[LeonMSPT]

Fire is weird sometimes... even does stuff it shouldn't sometimes. Generally, yep... burns up, like heat rises. Up, and out... hence investigators looking for "fan marks" at a fire scene to determine origin. Flammable liquid on a wall leaves "splash marks" and burns deeper into a wood floor where it is poured. The dog will sit right down there when it smells it too.

Chimney fires are the bane of wood burners. Fire is not an easy thing to manage, you can't "turn it off". The best you can hope for is to contain and control it. Sometimes you're lucky to do that. The mundane chores associated with burning wood are somewhat boring and repetitious. Cleaning pipes and chimneys, and never having had a fire, one might be tempted to skip it for a time. Loading, and opening cleanout doors and loading

Creosote also melts when it gets hot... and chunks break off and fall back into the cleanout area.

Picture a nasty cleanout area, with burning and melted creosote running and falling back into it...

That makes sense, I guess that is one way the fire could "fall or drip" back below the point of ignition" :zip:

 

[woodconvert]

I've heard it described as sounding like a 747 is landing in the front yard, or a 100 car train is chugging up a 10% grade past your house... or they can be small and you'll miss it entirely, and anything between.

The answer is, "You'll know if you have one that's big enough to worry about."

The one I had was obvious. I have a masonry chimney in a chimney (cement double wall if you will...) and between the 747 sound, the sound of stuff expanding rapidly and the tornadic cloud coming from the chimney...put two and two together. I threw in a Chimfex stick, put the fire out on the quick and had the fire department scan it for hot spots.

They suck, do what you can to avoid them.

 

[dolphins1lrb]

I've heard it described as sounding like a 747 is landing in the front yard, or a 100 car train is chugging up a 10% grade past your house... or they can be small and you'll miss it entirely, and anything between.

The answer is, "You'll know if you have one that's big enough to worry about."

The one I had was obvious. I have a masonry chimney in a chimney (cement double wall if you will...) and between the 747 sound, the sound of stuff expanding rapidly and the tornadic cloud coming from the chimney...put two and two together. I threw in a Chimfex stick, put the fire out on the quick and had the fire department scan it for hot spots.

They suck, do what you can to avoid them.

I too have had a chimney fire and it is a little on the scary side. My front lawn looked like a valcano erupted. You mentioned you threw in a Chimfex stick , someone told my husband about them but we can't seem to locate any. Can you tell me or can anyone tell me where to purchase them, I live in Western NY or can I purchase over the internet? Not that I plan on having another chimney fire but I like to be prepared.

 

[woodconvert]

I've heard it described as sounding like a 747 is landing in the front yard, or a 100 car train is chugging up a 10% grade past your house... or they can be small and you'll miss it entirely, and anything between.

The answer is, "You'll know if you have one that's big enough to worry about."

The one I had was obvious. I have a masonry chimney in a chimney (cement double wall if you will...) and between the 747 sound, the sound of stuff expanding rapidly and the tornadic cloud coming from the chimney...put two and two together. I threw in a Chimfex stick, put the fire out on the quick and had the fire department scan it for hot spots.

They suck, do what you can to avoid them.

I too have had a chimney fire and it is a little on the scary side. My front lawn looked like a valcano erupted. You mentioned you threw in a Chimfex stick , someone told my husband about them but we can't seem to locate any. Can you tell me or can anyone tell me where to purchase them, I live in Western NY or can I purchase over the internet? Not that I plan on having another chimney fire but I like to be prepared.

Unfortunately, I was told by a member here that....now you aint gunna believe it....the company burned down and never reopened and I also do not know of anyone making an equivalent. I've got some 8 year old one's and I wouldn't mind sending you one or two but I can't verify that they are still good so i'd rather not.

Sorry.