Chimney and Masonry

Most chimney fires are caused by a dirty chimney.

We often get asked, ‘What is the black stuff in my chimney?’ Well, anytime you burn wood, byproducts of combustion are formed, and those byproducts include creosote and soot. This is what you’re seeing – and it’s not good.

Soot needs to be removed from the chimney periodically, but the byproduct of woodburning we’re most concerned with when it comes to chimney fires is creosote.

Here’s why…

When you burn wood, the smoke produced by the fire contains unburned wood particles. The smoke cools as it passes through the chimney, leaving condensation on the walls of the flue lining in the form of creosote.

Creosote is a black or brown residue that can be crusty and flaky, tar-like, drippy and sticky, or shiny and hardened. At first, the buildup may be light and easy to remove. But with each fire you burn in your fireplace, creosote can build up, thicken, and glaze.

Naturally, creosote is corrosive and can damage the flue liner over time, but the real concern is its high flammability. Because creosote is formed from unburned wood particles, all forms of creosote are highly combustible. If the temperature within the flue is right, the creosote inside could ignite and cause a chimney fire.

And if you’re thinking the temperature in the flue would have to get intensely hot for creosote to ignite – think again. Creosote can ignite at temperatures as low as 451 degrees F (for context, a fire in your fireplace can burn in excess of 450 degrees F), and even just an 1/8” buildup of creosote is considered enough to cause a chimney fire.

Now, you may be thinking, ‘Big deal. Aren’t chimneys capable of withstanding a little fire?’ Well, the answer is no. Chimneys are meant to contain smoke, not fire.

In fact, if you have a fire in your chimney, within a matter of seconds or minutes, the tile liner could expand and crack, allowing heat and fire to access the attic, nearby walls, and other combustibles in the home through the brickwork. And that’s how a chimney fire rapidly spreads to become a full-fledged house fire.

But creosote isn’t the only thing responsible for dirtying the chimney and increasing the risk of a dangerous chimney fire…

Flammable blockages caused by birds’ nests and other debris, can also cause a chimney fire. And it’s not uncommon to find nesting materials in a chimney, either. In fact, even though many folks don’t realize it, an uncapped chimney is a welcoming place for birds, raccoons, squirrels, and other critters. These critters bring with them nesting materials, fur, feathers, and feces, all of which can cause a chimney blockage, dirty the flue, and lead to a chimney fire.

Like we said, most chimney fires are preventable. All you must do to prevent dirty chimney-related chimney fires is schedule annual inspections and cleanings as needed, and make sure your chimney is properly capped. Easy as pie.

So, what makes a chimney fire so dangerous and how bad is it?

Chimney fires are dangerous because they can rapidly spread to other areas of the home, putting those within the home at risk of injury or death. Not only that, but they can cause a lot of damage to the chimney itself.

Here’s a snapshot of the kinds of damage chimney fires can cause to your chimney, depending on which type you have:

• Masonry Chimneys – When a chimney fire occurs in a masonry chimney – whether the flue is an older, unlined type or tile lined to meet current safety codes – the high temperatures at which the fire burns (around 2000 degrees F) can melt mortar, crack tiles, cause liners to collapse, and damage the outer masonry material. Most often, thermal shock occurs, tiles crack, and mortar is displaced. All this damage provides a pathway for flames to reach the combustible wood framing of the home, which is extremely dangerous.
• Prefabricated/Factory-Built Metal Chimneys – In most jurisdictions in the U.S., metal factory-built chimneys that are designed to vent wood-burning stoves or prefab metal fireplaces must pass special tests or they can’t be installed. Most tests require the chimney to withstand flue temperatures up to 2100 degrees F, without sustaining damage. Even still, if there’s a chimney fire, these systems can be damaged. And unfortunately, when prefabricated, factory-built chimneys are damaged by a chimney fire, they must be replaced.
• Woodstoves – Woodstoves are made to contain hot fires. The connector pipes that run from stove to chimney, however, aren’t. They can’t withstand the high temperatures produced during a chimney fire and will warp, buckle, or even separate from the appliance and chimney because of the vibrations of air turbulence during a chimney fire. If a separation does occur, fire, heat, and smoke can transfer to other areas of the home. Additionally, like prefab chimneys, if the connector pipes on a woodstove are damaged, they must be replaced.

Another consideration is what a chimney fire can do to the home if it spreads. A rapidly spreading fire can cause extensive damage to walls, ceilings, attics, framing, and furniture. All those repair and replacement costs can add up quickly.

When you consider that a chimney sweeping and inspection cost around $150-450 and could potentially save you thousands or even tens of thousands of dollars in repairs, the smart choice is obvious: schedule routine inspections and cleanings for your chimney system.

Most chimney fires start inside the flue where there’s either creosote buildup or a flue blockage of some kind. As heat from the fire reaches these flammable materials, the creosote or blockage ignites, starting a chimney fire.

As we mentioned a little bit earlier, creosote has the potential to ignite at around 451 degrees F. Given that fires in fireplaces can reach temps exceeding 450 degrees F, it doesn’t take a miracle for creosote to reach the right temperature for a chimney fire.

Once a chimney fire is going, it can burn up to 2000 degrees F, which is hot enough to break and crack clay flue tiles and melt metal flue liners in mere moments. That’s why a small chimney fire can become a house fire in very little time.

Is a chimney fire always obvious? Good question. Chimney fires can burn explosively, making them noisy and dramatic enough to be detected by neighbors and people passing by. But not all chimney fires are obvious.

There are actually two types of chimney fires – the fast burn and the slow burn.

If you have a fast-burning chimney fire:

• You may hear loud popping noises or a low rumbling sound, almost like a freight train or a low-flying airplane.
• You’ll likely have large plumes of black smoke coming up through the top of your chimney, or maybe even sparks spraying out of your chimney top. (No, it’s not normal for sparks to come out of your chimney or for large black clouds of smoke to pour out the chimney top.)
• You may smell an intense, hot smell

If you have a slow-burning chimney fire, you may not know anything is happening. These types of chimney fires don’t get enough air or have enough fuel to be dramatic or visible. That’s why they often go undetected until a chimney inspection. But that doesn’t mean they’re any less dangerous.

They can reach high temperatures and lead to more heat transfer to nearby combustible parts of the house. And ultimately, they have the potential to cause just as much damage (if not more) than their spectacular cousins.

Check out the video below, where our Director of Education, Russ Dimmitt, explains.