How to Prevent Chimney Fires

What actually catches fire

Contrary to what many people picture, a chimney fire is not the fire in the fireplace climbing higher. It is the deposits lining the flue catching fire on their own. Wood smoke carries unburned gases, tar and water vapor. When that smoke cools on the flue walls, part of it condenses into creosote, a layer that thickens with every cool, smoky fire. Creosote is combustible, and once enough of it is present, a hot fire, a burst of flame from paper or a stray ember can light it.

When that layer burns, flue temperatures can climb far beyond anything the chimney sees during normal use. Clay tiles crack, mortar joints open and metal liners can warp. If heat escapes through those gaps, wood framing next to the chimney is exposed. Prevention, in practical terms, means two things: keep the fuel from building up, and keep the chimney able to contain heat if something does ignite.

The main causes, from most to least common

Across the housing we work in, from brownstones to twins to colonials, the same handful of causes come up again and again. Most are about how the fire is built and fed, not about the chimney itself.

• Burning damp or freshly cut wood, which steams and smolders instead of burning hot
• Turning a stove or insert down low for long, slow burns, especially overnight
• Going several seasons without an inspection or sweep while the fireplace is used regularly
• An oversized flue, often a large old masonry flue serving a smaller stove or insert, where smoke slows and cools
• A chimney on a cold outside wall that rarely warms up enough to keep smoke moving
• Burning paper, cardboard, wrapping or holiday greenery that throws flaming pieces up the flue

Burn dry wood, and burn it hot

The single habit that does the most is burning wood that is properly dry. Seasoned wood is lighter, has cracked ends and sounds hollow when two pieces are knocked together. The EPA's wood-burning guidance says wood burns most efficiently at a moisture content below 20 percent and recommends seasoning it for at least six months, stored off the ground with only the top covered so the sides can breathe.

How the fire is run matters almost as much. A bright, active flame sends hot smoke up the flue quickly, and hot smoke leaves less behind. A fire starved of air, with the damper or air control barely open, produces thick, cool smoke that coats the flue. With a stove, run it hot for a while after loading, then settle it down, rather than packing it full and shutting the air right away.

What never belongs in the fire

Paper and cardboard burn fast and light, and burning pieces can float up into the smoke chamber and flue, exactly where creosote sits waiting. Holiday wrapping, pizza boxes and a dried-out tree are common culprits after the holidays. Trash, painted or treated wood and anything with glue release chemicals and residue and do nothing good for the flue.

Manufactured fire logs are a separate question. Some are made only for open fireplaces, and the EPA advises checking a stove or insert's operating instructions before burning them there. Use them exactly as the package and the appliance allow, one at a time, and do not add them to a roaring wood fire.

Inspection and sweeping on a schedule

NFPA 211, the national standard covering chimneys and fireplaces, calls for a yearly inspection and cleaning when deposits have built up. The U.S. Fire Administration gives the same advice to homeowners: have heating equipment and chimneys cleaned and inspected each year by a professional. Timing it before the first cold snap means the chimney goes into the season clean.

How often a flue actually needs sweeping depends on use. A fireplace lit for a few weekend fires may need it less often than a stove that heats the house every day from November to March. The point of the yearly inspection is to measure what is there rather than guess.

Heating flues deserve the same attention. An oil boiler that shares a chimney with a fireplace, a common arrangement in older colonials and twins, sends its own soot into the stack, and a clogged base or cleanout can push exhaust back toward the boiler room. Asking for every flue in the stack to be looked at during one visit is simpler than discovering a problem in January.

Repairs that stop a small fire from becoming a big one

Even a clean chimney can have weak points. A cracked or missing flue liner lets heat and sparks reach the brick and any wood framing around it. Gaps in the smoke chamber, a rusted-through damper or a stove pipe that pushes too far into the flue all change how heat moves. A missing cap invites nests, leaves and debris that can catch fire, and it lets embers land on the roof.

Stoves and inserts connected to big, old masonry flues are a specific concern. A stainless liner sized to the appliance keeps smoke hot and fast, which reduces creosote at the same time as it contains heat.

• Cracked clay flue tile: heat and sparks can reach the brick and nearby framing; the fix is a liner repair or a new liner
• Oversized flue serving a stove: smoke slows, cools and leaves heavy creosote; the fix is a stainless liner sized to the appliance
• Missing or broken cap: nests and debris collect in the flue and embers land on the roof; the fix is a new cap with mesh
• Rough or open smoke chamber: creosote collects in gaps above the damper; the fix is parging the smoke chamber smooth

Signs the chimney is getting close to trouble

A few signs mean buildup is heavier than it should be. A strong tarry smell from the fireplace on humid days, black flakes dropping onto the grate, smoke that lingers in the room, a fire that is harder to keep lit and dark, shiny deposits visible with a flashlight above the damper all point toward a cleaning visit sooner rather than later.

Puffy, honeycomb-looking creosote, warped damper parts or cracked tiles found in the firebox can be signs that a chimney fire has already happened without anyone noticing. Those call for an inspection before the next fire, not after.

Around the fireplace itself

Fire prevention does not stop at the flue. The U.S. Fire Administration recommends keeping anything that can burn at least three feet from fireplaces and wood stoves. Use a screen or glass doors so embers stay in the firebox, put ashes in a metal can with a lid and set it outside, and make sure smoke alarms and carbon monoxide alarms on each floor are working.

If you would like the chimney checked before the season, our local crews inspect and sweep fireplaces, stoves and heating flues, and any repair that turns up comes with upfront pricing so you can plan around it.