Why are chimney fires dangerous?

Kind of a high-jack of another post asking if aluminum cans can get rid of the creosote that builds up in a chimney.

My question is, what makes a fire inside a chimney so dangerous? It seems to me that if a chimney is properly built with the aluminum and surrounded by brick. The fire should escape out the top of the chimney, eventually burning all of its fuel and dying out.

Back when i was a cook, and our char broiler would catch fire after a long night of cooking fatty foods, it would burn for 10-15 minutes and slowly die off, causing no harm to any surrounding equipment (i understand that cooking equipment is made to be hot, but so is a chimney)

A chimney fire can run a long time overheating the chimney which might allow it to ignite adjoining structure and allow smoke and gases into the home, or in the worst case collapsing the chimney. A chimney fire often emits a low of glowing embers that can set fire to the structure and nearby areas.

I think the basic answer is that a chimney fire can burn very very hot and radiate heat to the point of ignition into adjacent structural materials and/or onto roof shingles. The convection from a fire burning within the chimney column itself can produce terrific airflow that accelerates the fire even further – as the linked article says, it can sound like a freight train going by. I’ve heard sounds like that just when throwing a pizza box into a fireplace and I imagine it must be pretty frightening when the whole chimney column is ablaze.

I’ve never had a chimney fire, I just love fireplaces and have always been careful about the integrity of the fireplace masonry. I admit I haven’t been that careful about keeping the chimneys clean but I should be.

The problem isn’t as much a fire inside of a properly designed and constructed chimney, it is when there is damage to the chimney liner or the chimney itself. Any damage can allow the fire inside of a chimney to follow the damaged area out of the chimney and spread to the structure.

The other issue is that you can have a hot enough fire inside a perfectly good chimney that it is damaged by the fire, allowing the fire to leave the.chimney. That of course assumes you have a good chimney to start there are plenty of chimneys that have no damage but are not built to any recognized code. If the masonry is too thin, mortar incorrect, or there is inadequate clearance from combustible structure a fire can extend out of the chimney even without a break in the chimney.

A good indicator of an upcoming chimney fire is creosote leaking out of a chimney liner. A chimney fire will follow the path the condensed creosote took (heat equals pressure, the fire gasses look for the path of least resistance). Of course, the only time I have seen the creosote trail have been while opening walls chasing chimney fires that left the flue, so it may not be as helpful an indicator as it sounds.

Just hire a knowledgeable chimney sweep to brush the flue and inspect the chimney. Way easier than firefighters damaging the roof and/or liner when we drop our chains.

Chimney fires are especially dangerous because:
[ul][li]They often go undetected for quite a while. They are inside a chimney, not visible, and heat & smoke are normal & expected in the area. Often they’re only detected when they break out of the chimney into the house.[/li][li]They can get very, very hot (partly because of above). And they will pre-heat the structural materials next to the chimney, so those can burn faster when the fire reaches them.[/li][li]They are often deep inside the structure, so difficult to reach to extinguish.[/li][li]They usually extend through multiple floors of the house, and have a built-in access to oxygen.[/li][/ul]

They can get so hot that the mortar between the bricks (in older homes) breaks down and fire jumps out into your attic.

Also, they can be so hot that combustibles adjacent (like your roof) can become superheated and flame up, even when no actual fire has touched it.

Chimneys are double walled construction. It can be steel inside steel or clay inside brick. A clay flue is a series of hollow blocks mortared together. They’re subjected to heat and any shifting of the fireplace. They’re supposed to float independent of the surrounding brick structure but many are mortared to the brick at the top. This causes stress when the hot clay expands. Joints fail allowing fire to move into the brick chamber. The brick is decoration and not meant to withstand an intense fire. It would only take on small gap in the brick mortar to allow a fire to transition to the surrounding structure which is usually wood framing butted up against the chimney. A chimney fire fed by the air driven by burning wood in the fireplace is going to produce something akin to a blow torch on any holes in the chimney.

Steel chimneys are doubled walled. They often sit inside wooden box structures that approximate the look of brick chimneys. Hot fires can melt steel producing a small hole that grows over time.

Another reason they’re dangerous is because some people will try to put them out by throwing a bucket of water into the chimney. As a firefighter once put it: “If you see your neighbor climbing up a ladder with a bucket of water to put out the fire, your reaction depends on your relationship with that neighbor. If you really don’t like him, offer to hold his ladder for him, pass the bucket up to him and then gather all of your other neighbors, telling them that someone’s going to fly on a chimney.”