Creosote from a handful of fires a year, and why it still matters

What creosote is and where it collects

Wood smoke carries unburned gases, moisture and fine particles. EPA explains that when that smoke rises into a cooler chimney, part of it condenses and sticks to the inside walls. The deposit is creosote, which EPA describes as a highly combustible black or brown residue. It is not dirt that blew in from outside. Your own fires put it there.

One factory-built fireplace manual puts the same process in plain terms. When wood burns slowly, it gives off tar and other vapors that combine with moisture from the wood. Those vapors condense in the relatively cool flue of a slow fire and build up on the lining. The manual also warns that this deposit burns extremely hot once it ignites.

Creosote in a chimney collects wherever smoke slows down or cools. In a masonry fireplace that means the damper area, the smoke chamber above it and the flue lining. In a factory-built fireplace it coats the inside of the metal chimney pipe. A gas fireplace does not make wood creosote, although a wood fireplace later converted to gas logs should be cleaned of it before the logs go in.

Why a few short fires can leave more behind

EPA names the conditions that add to creosote. They are a restricted air supply, including a damper that is not open wide enough, wood that has not been seasoned, and a chimney cooler than normal. EPA ties that last one to very cold outside air, and notes that smoke lingering in the flue is more likely to leave creosote behind. An occasional fire can easily meet the first two.

Think about how a fireplace gets used a few nights a year. The flue starts the evening cold, because nothing has warmed it in weeks. The fire is lit for a dinner or a visit and allowed to die down within a couple of hours. Someone may close the damper partway to hold in cooled air. Much of that fire's smoke rises slowly through a flue that never fully warms, and that is when it condenses.

EPA's advice is to build hot fires, because a smoldering fire is neither safe nor efficient. A brisk fire with the damper fully open leaves less behind than a sluggish, smoldering one. Brisk does not mean oversized, though. The Superior manual warns that burning large amounts of scrap lumber, branches, paper or cardboard can overfire the fireplace.

Warming a cold flue at the start of the evening

The first minutes of a fire in a cold flue are the slowest and smokiest. The Superior fireplace manual addresses this directly. It suggests opening a nearby window and holding a rolled, lit sheet of paper up in the top of the firebox to warm the chimney. The fire itself goes in once the draft is going. Your own fireplace's manual is the one to follow, and some units give different steps.

The same manual lists what never belongs in the fire. Its list includes treated lumber, plywood, cardboard, colored paper and Christmas tree branches. Several of those turn up after a holiday gathering, the kind of evening an occasional fireplace is lit for. Keep them out, and the fire burns cleaner as well as safer.

Firewood that has sat in damp air

Wet wood spends much of its energy driving off water, and EPA notes that this makes more smoke and deposits more creosote. EPA recommends seasoning firewood for at least six months and testing it with a wood moisture meter before burning. Dry pieces also look darker and feel lighter than fresh ones, and two of them knocked together give a hollow sound.

How the wood is stored counts as much as its age. EPA's storage advice is to stack firewood outside and raised off the ground, covering the top of the stack but leaving its sides open so moisture can escape. A bundle bought on the day of a gathering may not be dry, so check it before it goes in. Never burn treated or painted lumber, trash or cardboard to get a fire going.

How the three forms look at the damper

Sweeps usually describe creosote in three forms, often called the creosote stages. The names describe what the deposit looks like, not a timeline every chimney follows. You can see the first surfaces with a flashlight when the fireplace is cold. Look at the damper plate and the smoke chamber walls just above it, without scraping or reaching up into the flue.

• Loose, powdery or flaky soot that a brush would lift off easily
• Dark, crunchy flakes with a tarry or bubbled look, which cling more firmly
• A hard, shiny coating like dried tar, sometimes running in drips, which routine brushing may leave behind

Why light use still calls for a yearly inspection

NFPA 211 sets a yearly inspection for chimneys, fireplaces and vents. EPA and fire officials also recommend a yearly professional inspection and cleaning for wood-burning equipment. The inspection decides whether cleaning is needed. A fireplace used twice a year may need little brushing, or it may carry a glazed patch from one smoky evening.

Idle months add their own findings. Nests, leaves and water can reach an open flue while it sits. The yearly visit looks at all of it together. We explain the findings before any cleaning is booked, and the cleaning is written up for your records afterward.

When brushing is not enough

Brushing removes loose soot and most flaky deposits. A hard glaze is different, because it bonds to the flue surface and routine brushing may leave it in place. Removing it may call for a treatment product used to its maker's instructions, or for heavier mechanical work chosen for the surface underneath. After a heavy glaze comes off, the flue lining under it deserves a close look before the fireplace goes back into use.

That kind of creosote removal is a separate job from a routine sweep, priced on its own line of the estimate. A routine chimney sweep handles the loose deposits. Either way we clean up as we go and haul the debris away, and the written estimate tells you before work begins which one the flue needs.

A chimney fire is an emergency

EPA warns that enough creosote can cause a chimney fire. If a fire in the flue ever roars, throws sparks from the top or pushes smoke into the house, get everyone out and call 911 from outside. Do not try to put it out from the roof. After any chimney fire, nobody lights the fireplace again until the flue has been inspected.

Habits that keep creosote down between visits

None of these habits replaces the yearly visit, but together they slow creosote buildup.

• Burn only dry, seasoned firewood, and check bought bundles before use
• Open the damper fully before lighting and keep it open until the fire is out
• Build a brisk fire of normal size instead of one left to smolder
• Burn only wood, never trash, cardboard or treated lumber
• Keep the cap and screen in place so debris does not slow the draft
• Arrange the inspection well before the evenings you plan to use the fireplace

Sources

• EPA Burn Wise frequent questions about wood-burning appliances: What creosote is, the conditions that add to it, including unseasoned wood that makes more smoke, chimney fire risk and yearly inspection and cleaning.
• EPA Burn Wise best wood-burning practices: Seasoning, moisture testing, firewood storage, hot fires rather than smoldering ones and materials never to burn.
• NFPA 211, Standard for Chimneys, Fireplaces, Vents and Solid Fuel-Burning Appliances: Yearly inspection of chimneys, fireplaces and vents, with cleaning when needed.
• Real Fyre GR47 vented gas log manual: A wood fireplace cleaned of soot and creosote and inspected before gas logs are installed.
• Superior WCT3000 wood-burning fireplace manual: A manufacturer's account of how slow fires form creosote, warming a cold flue before a fire, overfiring and materials never to burn.