Chimney Fires White Paper - Read This!

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Jan 23, 2007
Virginia coast
Hey, all, I've been a member for several years, but only visit occasionally in the winter heating season, so I don't know whether this has been posted already but IMHO it should be a sticky:

Apparently chimney fires burn down a LOT of houses every year, so I started researching them online, and found this very informative white paper on them:

Chimney Fires White Paper

I learned a LOT from this white paper and in my opinion, it ought to be "required reading" for anyone who uses a wood stove or fireplace.

Some interesting excerpts:

the purpose of regular chimney sweeping is not to prevent the occurrence of a fire but to limit its severity and effects should one occur. ... Significant chimney fires have been reported within a week of sweeping the flue. ..

The first external signs of a chimney fire are usually aural – a noisy inrush of air resulting from the very high draft created by extremely high flue gas temperatures. Those who have experienced it compare the sound to the roar of a jet plane taking off or a train rumbling through the dwelling. ..

One of the more dramatic and potentially dangerous effects of some chimney fires is back- puffing of smoke and sometimes flames from the appliance or venting system. This effect is caused by a series of rhythmic explosions of fuel-rich creosote gases in the confined area of the flue. As the gases burn, they may consume the available oxygen faster than it is supplied. When the concentration of oxygen in the gases reaches the lower limit of flammability, the flame is extinguished, but plenty of heat remains in the flue. Air will re-enter the flue and mix with the gases which then ignite suddenly and explosively. The result is immediate high pressure in the chimney which cannot be fully relieved out the top and smoke and flames may be driven through any opening in the flue or appliance. As the oxygen is again quickly depleted and flames are extinguished, the pressure drops suddenly, drawing in more air to initiate the next explosion. This cycle may be repeated several times a second. If the flue becomes blocked by fallen or expanded creosote during the fire, a large volume of smoke may spill back into the dwelling. ..

Almost without fail, a large volume of dense dark or black smoke will come out the top of the chimney. Sparks or embers of glowing creosote material may be drawn up by the strong draft and expelled from the chimney top. ..

A newly- ignited chimney fire does not engulf the entire chimney at once; in fact, it is likely that there is no point where the full length of the flue is on fire...

The standard advice given to homeowners, should they have a chimney fire, is to close the air controls on the appliance and any other sources of entry into the system. The idea is to deprive the fire of one of its essential elements: oxygen. Theoretically, this should result in an immediate halt to combustion. In practice, it rarely does. ..

Unless the system is unusually leaky, the closing of obvious openings will stop most of the flaming combustion in the chimney. Much of the heat generated by the fire will be retained, at least temporarily, in the chimney and will carry forward the pyrolysis of the creosote. Glowing combustion of pyrolized creosote will continue, being supported by the limited available air flow. Depending on the amount of glowing, sufficient heat may be released to continue pyrolysis of unburned creosote although the gases may not ignite. Creosote can continue to smolder in this manner for hours ...

... all of the [creosote] samples grew from 10 to 27 times their original volume with an average expansion of nearly 1400 percent. In other words, if the original deposit were evenly spread on the surface of chimney flue to a thickness of one-fourth inch, and completely burned as in this test, the expanded creosote residue would end up about three and a half inches thick. For an 8 by 12 nominal modular flue liner, this would essentially block the flue. ...

The standard advice given to homeowners to limit the entry of air is a sound first step in extinguishing a fire because this action will usually cut short the flaming phase. However, it may prolong the period of glowing fire and therefore should be followed by explicit efforts to actually extinguish the fire, preferably by the fire department

We had a chimney fire when I was a kid, and it was very scary. Now, whenever we burn the woodstove, we keep three things ready:

1. A bucket of sand.
2. A big piece of fireproof fiberglass cloth (we got some cheap used fiberglass curtains off of ebay)
3. A charged-stream water fire extinguisher (like they used to use in schools)...these can be recharged by the homeowner with water and pressurized with an air compressor through a Schrader (tire) valve.

In the event of a chimney fire, the plan is to:
a) immediately close the flue to starve the fire of oxygen
b) dump the bucket of sand onto the fire in the woodstove to smother it
c) use the charged-stream fire extinguisher to spray onto the sand-covered fire and into the flue throat. I have read that this will create a column of steam (water vapor) that will fill the chimney, absorb a lot of heat in the chimney and deprive the fire of oxygen.
d) jam the fiberglass cloth into the flue if it cannot be closed sufficiently to starve the chimney fire of oxygen
e) call the fire department...

Anyway, it is so cold in many parts of the country right now that I thought a lot of people who might not be experienced wood burners might be using their woodstoves and fireplaces, and that this information might help someone. I saved a copy of the white paper on my computer, and in that document, I went through and highlighted what I thought were the important parts, and I try to review it every year. It goes into great detail about various types of chimneys and how they should be constructed, how fires behave in various types of chimneys, etc etc. It's a long read but well worthwhile in my opinion.

Be safe out there!
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I found the burning creosote expansion part interesting. I did not know that.
Call 911 ...
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Reactions: begreen and bholler
I agree, but I think the need to ensure everything is safe would be helped by a visit from the FD. It's in addition to not letting it run away.
Nearest volunteer FD is 30 mins out. I own an industrial fire extinguisher. Going to probably take @bhollers advice and not just let it go.
Calling 911 for them to come and check for any hidden hot spots is definitely needed as well. That doesn't mean to just sit there doing nothing while you wait. But still do it
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I agree, but I think the need to ensure everything is safe would be helped by a visit from the FD. It's in addition to not letting it run away.

Calling 911 for them to come and check for any hidden hot spots is definitely needed as well. That doesn't mean to just sit there doing nothing while you wait. But still do it
Thats the plan always call but as you have said @bholler there are things to do in the meantime.
Keep more than one fire extinguisher too. I have a few stashed around here in closets and behind furniture. And check them once in a while. I’ve used extinguishers at work a few times, and they don’t take long to empty.

For snuffing out a mock training fire in an old steel wheel rim, they work great. In real life depending on the fuel it may take more than just a quick puff, and if there is a secondary fire, you may wish you had a second or third.
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I'd say evacuating the house and calling 911 are the first order of business. Have a fire extinguisher in the room, preferably close to the exit door and use that after people and pets are out of the house.
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