Preventing Chimney Fires

What a chimney fire actually is

When wood burns, the smoke carries water vapor, gases, tar fog and tiny unburned particles up the flue. The Chimney Safety Institute of America (CSIA) explains that as these byproducts reach the cooler upper chimney, they condense on the flue walls. The residue is called creosote, and it can be flaky, sticky and tar-like, or hard and shiny.

Creosote is fuel. A chimney fire happens when the layer on the flue walls catches. CSIA describes the dramatic version as loud cracking and popping, dense smoke and an intense hot smell. It also points out that most chimney fires go unnoticed: a slow-burning fire without much air can still reach very high temperatures and damage the liner and masonry without anyone in the house knowing.

That second point is why prevention matters more than detection. You cannot count on hearing it.

Three conditions line up before a flue ignites

It helps to think of a chimney fire as three conditions arriving together:

1. A deposit to burn. Creosote, soot mixed with tar, or debris such as a nest.
2. Enough heat to light it. Usually a hot fire below a flue that already has buildup, or burning material that flares up fast.
3. Air to keep it going. An open damper and a drafting chimney supply it.

You cannot remove the air and heat without giving up the fire, so prevention works mostly on the first condition: keep the deposit thin, and avoid the sudden heat spikes that ignite whatever is there. Each lever below acts on one of those two things.

Lever 1: What goes into the firebox

The fuel decides how much residue the smoke carries. CSIA notes that unseasoned wood produces cooler smoke, and cooler smoke condenses into creosote faster.

• Dry wood. The U.S. EPA's Burn Wise program says wood burns best below 20 percent moisture and recommends checking it with a moisture meter. Our firewood guide covers testing and storage.
• No paper as fuel. The U.S. Fire Administration (USFA) tells homeowners not to burn paper in a fireplace or wood stove. Loose paper burns fast and hot, and burning pieces can lift up the flue.
• No trash or treated wood. EPA lists cardboard, plastics, foam, glossy printed paper, and coated, painted or pressure-treated wood among the things that should never be burned.

Small amounts of dry kindling or natural fire starters to get the fire going are fine. The issue is using fast-flaring material as fuel, or adding wet wood that smolders.

Lever 2: How you run the fire

The same wood can leave more or less residue depending on how the fire is run.

• Give it air. CSIA advises opening the damper fully and not closing glass doors on a fire that needs air. A fire starved of air smolders and makes more creosote.
• Keep the load reasonable. CSIA warns against overloading the firebox. Fairfax County Fire and Rescue puts it simply: use only enough fuel to keep the fire at the temperature you want, and avoid roaring fires.
• Avoid long smolders. EPA says that for most appliances a smoldering fire is neither safe nor efficient. Turning a stove down to burn all night is a common way to build creosote quickly.

The pattern to avoid is a cycle of slow, smoky fires followed by one very hot one. The slow fires lay down the fuel; the hot one supplies the heat to light it.

Lever 3: The appliance and the flue

Some chimneys collect creosote faster than others for reasons that have nothing to do with burning habits.

• Flue size and temperature. Smoke cools faster in a flue that is much larger than the appliance needs, or in a chimney on an outside wall exposed to cold air. Cooler smoke condenses sooner. The fix there is usually a properly sized liner. See liner options.
• Stoves and inserts. USFA advises using a wood stove that has been tested by a qualified laboratory and keeping its doors closed except when loading or stoking. An insert or stove connected to a flue without a proper connection or liner can deposit creosote in places that are hard to reach. See how a wood stove should be vented.
• Damage already present. Cracked tiles and open joints let heat reach the surrounding masonry if a fire does start. An inspection is the only way to know.

Burning that happens outside the routine

Many risky fires are not everyday fires. They happen on the days when something unusual ends up in the firebox.

• After the holidays. Wrapping paper, boxes and printed packaging are tempting to burn. EPA lists cardboard, the colored ink on magazines, boxes and wrappers, and other household trash among things not to burn. Paper and cardboard flare fast, and burning scraps can be carried up the flue.
• Cleanup projects. Leftover trim, pallets, plywood offcuts and old furniture often carry paint, glue or preservatives. EPA warns against burning coated, painted and pressure-treated wood.
• Starting a stubborn fire. Never use gasoline, lighter fluid or other flammable liquids in a fireplace or stove. Damp wood that will not catch is the real problem to fix.
• Leaving it unattended. USFA advises putting the fire out before going to sleep or leaving the house.

Lever 4: Removing deposits before they count

CSIA's shorthand is that clean chimneys don't catch fire. USFA recommends that a professional inspect and clean the chimney each year. The inspection tells you how much has built up; sweeping removes it.

How often a flue needs sweeping depends on how much you burn and how the fire is run, which is why one household may need it more than once a season and another much less often. Our guide on how often to sweep explains the measurement sweeps use. If you would like to know what actually happens on the visit, see what happens during a chimney sweep.

How the risk changes over one winter

Here is a hypothetical example, for illustration only. A household burns a few fires a week from November through March in an open fireplace.

• November. The flue was swept in the fall. Deposits are thin. The main risks are the first fire of the season and anything that nested over the summer. See the first fire of the season.
• January. Deposits from two months of fires are on the flue walls. If the wood still in the pile is damper than the early-season wood, or the fires get bigger on cold nights, this is when the three conditions are most likely to line up. Check the wood with a meter and keep fires moderate.
• February. A mid-season look is worthwhile if the household burns heavily. A flashlight look past an open damper cannot measure creosote; a sweep can.
• Spring. Leaving deposits in the flue over the summer brings odor in humid weather and means the next season starts with a head start on buildup.

If you think the chimney is on fire

After any suspected chimney fire, do not use the fireplace or stove again until the flue has been inspected. Our guides on signs of a chimney fire and what to do afterward cover the next steps.

Questions homeowners ask

Sources

1. CSIA: Preventing Chimney Fires
2. U.S. Fire Administration: Fireplace and Wood Stove Safety (handout)
3. U.S. Fire Administration: Heating Fire Safety
4. U.S. EPA Burn Wise: Best Wood-Burning Practices
5. Fairfax County Fire and Rescue: Fireplace and Ashes Safety Tips

This guide is general information. It can't diagnose a specific chimney; an inspection can.