Preventing Chimney Fires Via Annual Inspections and Cleaning
Text & photos by Neil Soderstrom

Many thousands of chimney fires are reported annually. Thousands more go unreported because residents allow them to burn out on their own. Still other chimney fires go undetected because residents aren't home or don't hear the quieter-burning variety. Most chimney fires occur in flues serving fireplaces and woodstoves--the result of the ignition of creosote residues that have condensed on flue walls.

What's So Bad about a Chimney Fire?
Most masonry chimneys have a hollow clay-tile flue liner designed to route hot smoke and gases out the top. However, clay liners can crack or shatter under stresses from earth tremors, violent storms, freeze-thaw cycles--or a single chimney fire.

During a chimney fire, flue temperatures can rapidly reach 2000*F, compared to normal operating temperatures of less than 400*F. This superheating of flue tile causes it to expand faster than the exterior, and the resulting thermal stress can crack and shatter tile sections. Superheating can also crack, warp, and buckle the steel flue of a prefabricated fireplace.

In addition, superheated chimney walls supporting the flue may crack, allowing flames and hot gases to reach house structure. And even if flames breaching the flue don't also breach the chimney's walls, they can heat the chimney and nearby house structure enough to start a house fire.

Our Chimney Fire When we moved to our present farmhouse, we decided to make its open brick fireplace more efficient. We hired a mason to reduce the depth of the firebox by sealing it with brick and leaving an opening for a stovepipe. On the brick hearth we placed a handsome, green-enameled, "airtight" wood stove with glass doors. That wood stove made our new home cozy, radiating generous amounts of heat, while consuming far less warm room air than the former fireplace did.

During our first winter, we burned scarcely a cord of firewood--a stack 4 x 4 x 8 feet. We'd heated our previous home primarily with wood, burning three to four cords per year without an excessive buildup of creosote. So I reasoned that the flue tile could go an extra year without cleaning.

One February night during our second winter, I was preparing the stove for a slow overnight burn. As usual, I'd filled the stove with a fresh load of wood and let the flames get going. But as I was about to close down the air intake dampers to reduce the rate of combustion, I heard what I mistook for a strong wind outside, or maybe a low-flying jetliner. No, this was a steady, hurricane roar coming from inside the flue. "It can't be!" I thought as I dashed outside to the base of the chimney and opened its iron clean-out door.

An orange-red glow reflected from the roaring fire above. "Chimney fire!" I gasped. The sudden in-rush of air through the clean-out door heightened the roar. I slammed the door shut and dashed back inside to the wood stove. There, I tried to reduce flue draft by removing the burning logs with fireplace tongs. One by one, I dropped the still-flaming logs into an ash bucket. In two trips I lugged them outside and tossed them into the snow. This removal process, of course, smoked up the living room, setting off the smoke alarms. Amid the smoke and cacophony of alarms, I closed the stove vents as best I could, hoping to reduce airflow to the flue.

Outside again, I gazed up at the chimney. Flames like jetliner exhaust struck the large stone cap that served as the flue's rain cover and splayed sideways against the clear starry night. The chimney now resembled an inverted rocket spewing embers onto the roof (fortunately snow covered).

I ran back inside and dialed 911. Within what seemed just a few minutes, our snow-lined country lane was lit by the roof flashers of two fire engines and a dozen or more vehicles of volunteer firefighters in helmets and heavy water gear--already pulling hoses toward our house. While one firefighter went up into our attic to monitor heat from the chimney's exposed brick and another moved his palms along the bedroom wall adjoining the chimney, a third firefighter climbed to the peak of the snowy roof, where the chimney still spewed flame and ember. Using gauntlet gloves, the guy on the roof lowered a chain into the flue, gently rattling it to loosen encrusted creosote.

After another fifteen minutes or so, the roaring subsided as the flames abated. And the firefighters departed. The next morning I lowered a light into the flue to inspect for damage. The flue looked quite clean except for scorch marks. The flue tiles near the top appeared to be completely intact--no visible cracks, no chipped surfaces. Below, upon opening the clean-out door, I found no fallen mortar or flue shards--just some creosote debris. We'd been lucky.

Mistakes that Caused Our Fire
During the fire, I learned a bit from the fire chief when I asked if "we" shouldn't try to put out the fire. He explained that cold water would likely cause the superheated flue tiles to shatter and might cause the overheated chimney to crack as well. If the chimney looked sound, as ours did, he explained, he usually let the fire burn out. "When it's over," he said, "you'll probably have a clean flue. But in the future, get that thing inspected and cleaned annually."

That was good advice. Based on my recent discussions with Royal Edwards, Technical Director of the Chimney Safety Institute of America (CSIA), I learned about additional mistakes that caused our fire.

• A small, hot stovepipe that vents into a far larger flue results in a longer "residence time" of emissions inside the flue. This slow exit of emissions allows more combustibles to condense on flue walls. The National Fire Protection Association (NFPA) recommends that the ratio of flue cross section to stove collar not exceed 2:1. Yikes! Our ratio had been 4:1. And that partly explains why the smaller flue at our previous house collected far less creosote.
• Cold flue walls of exterior chimneys cause more condensation of creosote than flue walls of chimneys rising through a house interior. To lessen condensation, Edwards recommends that flue liners themselves be insulated. Also, stainless steel is a much better liner than clay tile.
• When buying our stove, I had succumbed to advertisements boasting that the stove was an "all-nighter." However, to achieve all-night burns, such stoves have to be "air-tight"--allowing you to reduce the air supply for the required slow burn. Such slow burn rates lead to low flue temperatures and the slow exit of emissions that promote creosote buildup. Instead of slow-burning, Royal Edwards recommends ensuring that flue temperatures remain between 350*F and 1000*F. You can monitor this by installing a stem thermometer in the stovepipe near the flue. (Flue temperatures higher than 1000*F are more likely to ignite flue creosote and start a chimney fire.)
• As to my own untrained eye when inspecting our flue after the chimney fire, Edwards cautions that an unnoticed hairline crack inside a cold flue can expand up to a half inch during the next chimney fire. He notes that today's chimney firms can perform video inspections of flue linings so that stress cracks and gaps in mortar joints can be spotted before they cause a problem.
• Edwards makes several other suggestions that can reduce creosote buildup: (1) Burn well-cured hardwoods rather than "green" woods, which have a far higher moisture content and waste energy while burning off steam that condenses on flue walls. (2) Use wood stoves with catalytic combustors that help ignite smoke before it leaves the stove. (3) Spray anti-creosote catalyst -- "ACS" -- onto firewood to help protect flue walls.

Chimney Inspections & Cleaning
While many homeowners clean (sweep) their chimney themselves, most would benefit from the expertise offered by professional chimney firms, such as Northeastern Masonry & Chimney Services, in our region.

Northeastern's owner, Joe Shear, explains that the sophistication of chimney inspections and cleaning has increased tremendously since the 1970s. Before that time, sweeps tended to be part-timers who used a rope to lower a bag of chains into the flue and slam it around. While this knocked out squirrel nests and sometimes loosened some creosote, it also damaged flue tiles.

Northeastern is staffed by school-trained and certified professionals who not only inspect and clean chimneys, they also perform video inspections of flues, make chimney repairs as needed, and can reline chimneys with stainless-steel flues. Website: www.northeast ernchimney.com. Phone: (845) 454-4455.

Aside from cleaning, Joe Shear suggests that one of the best preventive-maintenance measures is a stainless-steel cap over each chimney flue. Here's why: Flue caps keep rainwater out. Also, the integral spark-arrestor grating prevents large sparks and embers from flying out. And very importantly, that grating prevents birds, squirrels, and raccoons from nesting inside and impeding airflow.

Some people mistakenly use waterproof paints on chimneys. But paints trap moisture inside chimney walls. In cold weather, this usually results in ice damage: cracked or even "exploded" mortar joints. Instead, chimney pros use a breathable, clear masonry sealant that allows the release of interior water vapor, while also retarding penetration of rain-driven water.

Although wood-burning appliances cause the creo-sote buildups that generate most chimney fires, each type of heating fuel creates its own unique flue damage. Joe Shear says, for example, that flues serving oil and gas burners can suffer significantly from the corrosive action of low-temperature emissions on flue walls. "In fact," he says, "my staff and I find more oil- and gas-furnace flues in unsafe condition than wood-burning flues." So in addition to your wood-burning flue, it's smart to have other flues inspected too.

Liability insurance: Before allowing anyone to climb up on your roof, ask them to have their insurer provide you certificates of insurance, verifying current worker's compensation coverage and business liability insurance.

More about Flue Liners
It's almost impossible to replace cracked and broken flue tiles without tearing the chimney down to the work area. A better option is to reline the chimney with either stainless steel or poured concrete.

Flue liners function much like the glass liner in a Thermos bottle. The resemblance to a Thermos bottle increases with stainless-steel flue liners. An insulating air space encircles the liner, protecting the chimney structure from excessive heat. The liner also prevents corrosive emissions from attacking the chimney structure. Stainless steel comes either as flexible or rigid tubing. Installers can snake flexible tubing (like an anaconda) into your old flue and insulate around it with mineral particles. Rigid steel tubing can have an insulating wrap. Costs for stainless flues range from $1,000 to $5,000, depending on flue dimensions.

Alternatively, installers can create a concrete flue liner by various proprietary methods. The advantage of poured flues over stainless is that they can improve a chimney's strength. But the quality of the installation depends on the skill of the installer. More labor--intensive than stainless flue installations, poured flues usually cost about 30 percent more.

Whether stainless or poured, be sure your installer uses a "listed" liner system, tested in a lab to Underwriters Laboratories (UL) 1777 chimney liner standard. A lifetime warranty lends further assurance. And, before having a new flue installed, obtain okays from local code authorities because permits and an inspection may be required. As to codes, a widely used reference is the National Fire Protection Association's technical document NFPA 211: Chimneys & Fireplaces (50 pages, $36), available from the Chimney Safety Institute of America, (317) 837-5362. For excellent supplemental info, consult www.csia.org.

Neil Soderstrom is a gardening writer and photographer based in Wingdale, NY, also found at www.agpix.com/soderstrom.