What Is Creosote Buildup in a Chimney?

Where it comes from

Wood does not burn in one step. Heat first drives water out of the log, then releases a mix of gases and tar vapors, and only when those gases mix with enough air at a high enough temperature do they burn as flame. Whatever does not burn leaves the firebox as smoke: water vapor, unburned gases, tiny droplets of tar and fine carbon.

As that smoke rises it gives up heat to the flue walls. When the walls are cool enough, part of the vapor turns back into liquid and clings to them. The liquid dries into a crust, and the next fire adds another layer. Creosote is simply the record of every bit of smoke that cooled before it got out of the chimney.

Why some chimneys coat up faster

Two houses burning the same number of fires can end the winter with very different flues. Anything that makes the smoke cooler, slower or wetter makes more creosote. From Long Island capes to Philadelphia twins, the causes below turn up roughly in this order.

• Wood that is not fully seasoned, which spends its heat boiling off water and smolders instead of flaming
• Small, slow fires, or a damper or stove air control left partly closed to make a load last longer
• A flue too large for the opening or stove, so smoke moves slowly and cools on the way up
• An exterior chimney on a colonial or cape, which stays cold all winter
• A wood stove or insert turned down overnight, which produces cool, tar-heavy smoke for hours
• A short chimney or one blocked by debris, which draws weakly and lets smoke linger

Where creosote collects

Deposits are rarely even from top to bottom. On an open fireplace the heaviest buildup is usually in the smoke chamber, the sloped masonry funnel above the damper, and on the smoke shelf and damper plate. Those surfaces sit just above the fire but out of the direct flame, and smoke slows there as it squeezes into the flue.

The top few feet of the flue are another trouble spot, because that section is the coldest and the smoke has given up most of its heat by the time it arrives. A cap's mesh screen can clog with creosote too, which chokes the draft. On wood stoves and inserts, the connector pipe and the first stretch of liner above the appliance tend to coat fastest.

Rowhouse and brownstone chimneys add one more spot. Their flues often jog sideways on the way up to make room for neighboring flues in the same stack, and the bend in an offset flue is where smoke slows, cools and drops deposits. Brushes pass through bends less cleanly than through straight runs, so offsets get extra attention during a cleaning.

Three forms, from easy to stubborn

Sweeps describe creosote in three stages. The first is dry and sooty and brushes off easily. The second is flaky and crunchy, shiny black with a puffy look, and takes more effort. The third is glazed: a hard, glassy coating that can look like dripped tar or melted plastic and can be thick enough to narrow the flue.

Glazed deposits are the hardest to remove and burn the hottest when they ignite. They are most common in stoves and inserts that are run low and slow, and in fireplaces where damp wood is burned behind half-closed doors.

Why a coated flue is a problem

Fire risk comes first. Creosote is combustible, and a hot fire, a burst of burning paper or a flare-up from dry kindling can ignite it. A chimney fire can be loud and obvious, or it can burn quietly and leave behind cracked flue tiles and warped metal that nobody notices until an inspection.

Heavy deposits also narrow the flue and roughen its walls, so the draft weakens and smoke rolls back into the room. Creosote holds moisture and is acidic, which works on clay tile, mortar joints and the steel of the damper over time. And in warm, humid weather it produces the familiar smell of campfire, asphalt or barbecue drifting out of the fireplace.

What you can check from the hearth

Once the fireplace has gone cold, open the damper and shine a strong flashlight up past the damper into the smoke chamber. A thin, matte, dusty black coating is normal on a fireplace in use. Shiny, bubbly, flaky or dripping deposits mean the flue is due for cleaning, and a glassy coat means it needs attention before the next fire.

Other clues are easy to notice without climbing anything.

• Black flakes or curls falling onto the grate or hearth
• A strong smoky or tar smell from the fireplace on humid days
• Fires that are harder to start or smoke at the opening more than they used to
• A cap screen that looks dark and clogged when viewed from the ground with binoculars
• Dark, sticky staining on the glass doors or around a stove door gasket

How a sweep removes it

First- and second-stage creosote comes out mechanically. The opening is sealed and a vacuum runs at the hearth to keep dust out of the room, then brushes sized to the flue are worked through its full height and the smoke chamber, shelf and damper are scraped and brushed by hand. Debris is collected from the base and the cleanout.

Glazed creosote needs more. Rotary cleaning tools on flexible rods can cut it back, and chemical treatments applied over a period of fires can change it into a drier form that brushes out. Heavy glaze may take more than one visit. Once the flue is clean, the liner, damper and smoke chamber can be properly examined, which is often when cracks hidden under the deposits come to light.

Under NFPA 211, the national chimney standard, an inspection is due every year and cleaning whenever deposits have accumulated. How often that cleaning is due depends on how much and how you burn, not only on the calendar.

Burning habits that keep it down

Use wood that has been split and dried until it is light for its size, checked with cracks at the ends, and rings rather than thuds when two splits are struck together. A moisture meter pressed into a freshly split face gives a firmer answer. Build fires that burn bright with active flame, open the damper all the way, and avoid choking the air down to stretch a load.

On a stove or insert, run the fire hot for a while after each reload before turning the air down, and follow the manufacturer's instructions for overnight burning. If a fireplace or stove coats up quickly no matter how it is used, the flue may be oversized or too cold for it, and a properly sized, insulated liner can change how it burns.