Creosote buildup is what wood smoke leaves behind when it cools before reaching the top of the flue. Tar and gas in the smoke condense on the wall, and what starts as soft soot you can brush ends as a hard black glaze that burns.
What Causes Creosote Buildup
Wood smoke carries unburned gas, tar and water vapor. Where the flue is cool enough, that mixture condenses on the wall as creosote. A hot, fast, straight flue gives it almost nowhere to land. A cool, slow or oversized one gives it plenty.
Four things cool the smoke or slow it down, and a flue that gives trouble usually has more than one:
- Wet or unseasoned wood, which spends the fire's heat boiling off water instead of burning the smoke.
- A stove damped down to smolder overnight, which makes a lot of smoke and very little heat.
- A flue wider than the appliance needs, so the gas spreads out, slows down and cools on the way up.
- An uninsulated run up an outside wall or through a cold attic, where the wall is colder than the smoke.
Gas appliances leave almost none of it, which is why this page is about wood. A wood stove, an open fireplace and a masonry flue collect the same deposit and differ only in how fast.
The Three Stages of Creosote Buildup
Sweeps sort the deposit into three stages by how hard it has set. Stage one brushes off, stage two flakes off with effort, and stage three has fused to the wall as a shiny black glaze that a brush will not touch.
| Stage | What the deposit looks like | What it means for you |
|---|---|---|
| One | Dry, dull, loose flaky material, close to ordinary soot | Normal for a flue in use. A brush of the right type clears it |
| Two | Darker, thicker, hard crusty creosote in shiny black flakes | The flue is running cooler than it should. Find the cause as well as sweeping it |
| Three | A tarry or glassy layer fused to the wall, sometimes running like candle wax | A sweep's brush will not lift it. Treat it as a professional job and a reason to inspect the flue itself |
A flue is rarely all one stage. The deposit is hardest where the gas ran coolest, which is the top of the run and any section out in the weather.
How Can You Tell Creosote Is Building Up?
Most of a flue is out of sight, so judge it by what reaches the room. A smell, a darker deposit than last season and a fire that has become harder to light arrive first.
- A sharp, tarry smell from the stove or fireplace, strongest on a warm damp day when the flue is cold.
- Black shiny flakes dropping into the firebox or onto the stove top.
- A deposit that feels sticky on a cold flue rather than dry and dusty.
- A fire slow to catch that pushes smoke into the room at lighting, where it used to draw straight away.
- Soot marks around a joint, a tee or the appliance collar.
- Dark staining on the outside of a masonry chimney, near the top or around the crown.
- A crusted spark screen at the top, which shows what the rest of the flue has collected.
None of that tells you how thick the layer is. Only a look up the flue does, which is what an inspection is for, and our guide to cleaning a chimney flue covers the brush types, the method and the point at which the job belongs to a sweep.
How Much Creosote Buildup Is Too Much?
Any deposit that is shiny, hardened or glazed is already too much, whatever its thickness, because a brush will not remove it. Soft dull soot is a cleaning job. A glaze is a professional job, and sometimes the end of that flue.
Thickness is measured at an inspection, not guessed at through a cleanout door. The practical test is simple. Dull and soft, and a sweep with the correct brush will clear it. Shiny, hardened or visibly narrowing the flue, and the appliance stays unused until someone has looked at the whole run.
Why Creosote Buildup Is Dangerous
Creosote is fuel, and it sits inside the one part of the house designed to stay hot. A flue lined with it can catch, and chimney fires burn fast and far hotter than the fire in the appliance below.
There are three separate problems, and the second one feeds the first:
- Chimney fires. A loud roaring from the flue, flames or heavy sparks at the top, and a stove pipe suddenly too hot to stand near. Get everyone out and call 911.
- Blocked airflow. A thick layer narrows the flue, a narrower flue draws less, a weaker draft runs the fire cooler, and a cooler fire lays down more deposit. That loop is why a neglected flue gets dirty faster every season.
- Fumes and smell. Creosote smells because it still holds volatile tar. A smell in the room means air is coming down the flue rather than up it, so treat it as a draft problem and an inspection prompt, never something to mask.
Our guide to a wood stove chimney fire sets out what to do while one burns and the inspection the chimney needs before the stove is lit again.
What Creosote Does to a Stainless Liner and to Class A Pipe
Stainless steel does not mind soot. What it minds is acid and heat. Creosote holds acidic condensate against the steel through the damp months, and a chimney fire takes the whole system far above the temperature it is listed to run at day to day.
Our Class A insulated chimney pipe is listed to UL 103, the standard for factory-built chimneys, and is rated for 1,000 degrees Fahrenheit, or 538 degrees Celsius, in continuous use. Its insulation keeps the flue gas warm enough to leave instead of condensing, and it is what the clearance in that listing depends on:
Class A insulated chimney pipe, when UL 103 HT-listed for wood-burning, maintains a minimum of 2 inches clearance to combustibles per the standard listing. Always verify the exact clearance specified by the manufacturer's UL listing for the specific product.
A system that has been through a chimney fire has been outside the conditions that listing describes, so it gets inspected before it is used again rather than simply swept.
A flexible stainless liner is thinner than Class A pipe by design, because it has to bend down a masonry chimney. Acidic deposit on a thin wall is what shortens its life, so controlling the deposit does more than any steel grade can. The flexible liner in our kits is 316Ti stainless, suitable for wood stoves, multi-fuel stoves and open fires.
What Reduces Creosote Buildup
Burn hotter and drier, and give the smoke a short warm route out. Prevention is free; removal costs a sweep, and in the worst case a new flue. Work down this list in order, because the first three cost nothing at all.
- Burn dry, seasoned wood. Wet wood spends the fire's heat boiling water off, and that heat is what carries the smoke up and out.
- Stop damping the stove down to smolder. A small hot fire leaves less behind than a large slow one, and it puts more heat into the room.
- Run the air controls the way the manual that came with the stove sets out. Those settings assume a flue sized and insulated for it.
- Match the flue diameter to the appliance. An oversized flue slows the gas and cools it, and no amount of sweeping fixes a size problem.
- Insulate the cold parts of the route. An outside wall and an unheated attic are where the deposit builds first.
- Have the whole run inspected, and cleaned when the inspection calls for it rather than on a calendar.
Three things people try that do not work:
- A deliberately fierce fire of aluminum cans, potato peelings or scrap lumber, lit to burn the flue clean. Nothing on the fire lifts a glaze off the wall, and overfiring a loaded flue is one of the ways a chimney fire starts.
- A solvent poured or sprayed down the chimney. No liquid a homeowner can send down a flue takes off a hardened deposit; that layer comes off mechanically or not at all.
- A chemical treatment used in place of sweeping. Our chimney flue cleaning guide is blunt about this: a product can dry a deposit so a brush has something to bite on, but the flue still has to be brushed and inspected.
What You Are Likely to Be Buying
Creosote itself has nothing you buy against it. The products you are likely to encounter fall into three groups: brushes and rods for the flue you have, chemical treatments that only ever assist a sweep, and replacement flue where the chimney is the cause.
Complete flexible liner kits run about $500 to $900 on our stainless steel chimney liner kits page, depending on diameter, length and insulation, and each kit arrives with an appliance connector or a tee to suit the stove. Insulated Class A sections are bought by the length instead, so a cold outside run can be improved on its own.
If you would rather not work that out, our free flue-design service will size it from your appliance, your building and the route the flue takes, and send back a parts list that fits. You can also talk it through on 888-834-0880.
When Is a Liner or a Pipe Past Saving?
Cleaning fixes a dirty flue. It does not fix one that has been damaged, has lost its shape, or has been through a chimney fire. Those are replacement questions, and an inspection answers them rather than the view from the room.
Replace rather than clean when any of these is true:
- The deposit is glazed and has resisted sweeping, so the surface underneath cannot be inspected properly.
- The liner is holed, split, crushed, or has pulled apart at a joint.
- The system has been through a chimney fire or another overheating event.
- The flue is the wrong size or the wrong type for the appliance, so the deposit will simply come back.
Age on its own is not a reason. Our guide to how long a chimney liner lasts sets out what to expect from each type, and everything here carries a limited warranty that holds whether you fit the system or a contractor does, provided the supplied instructions are followed.
Need More Information?
Sizing, clearances and flue temperatures are answered on our class a chimney questions page.
Each route out is taken step by step in the wood stove pipe installation guides. And if you would rather have someone look at your setup with you, talk to our technical team or call 888-834-0880.
