Creosote Stages 1, 2 and 3: What Each One Means for Your Flue

By Mark Jason, owner of NWI Chimney Sweep · 34 years in the trade

Every wood fire sends water vapor, gases and tiny tar droplets up the flue, the passage inside the chimney. When that smoke touches a surface cool enough, the tar condenses and sticks. What it turns into depends on how hot the flue was and how much air the fire had. That is why two neighbors who burn the same cord of wood can end up with very different chimneys.

Stage 1: soot

Stage 1 creosote is dull, dusty soot that flakes off on a finger and comes away with an ordinary chimney brush. It is what a healthy open fireplace produces: mostly carbon with a little tar binding it. A routine chimney sweeping visit takes it down to bare flue tile. It still burns, though, and NFPA 211 calls for cleaning once any deposit reaches 1/8 inch.

Stage 2: hard flakes

When smoke lingers in the flue instead of moving briskly out, more tar condenses and the deposit hardens. I find stage 2 mostly on fireplaces burned with the glass doors closed and on wood stove inserts damped down overnight. A flat wire brush skates over it. Removal takes a rotary head spun by a drill, working a few feet at a time, and it takes longer than a stage 1 sweep.

Stage 3: glaze

Stage 3 creosote is a glazed, tar-like coating that brushing will not remove and that burns far hotter than soot. It forms when flue temperatures stay low for long periods: green wood, a fire choked for a slow burn, or an appliance venting into a flue far too large for it. It can look like black enamel on the tile, or like tar that ran and hardened into icicles. The Chimney Safety Institute of America puts chimney fire temperatures at around 2,000 degrees Fahrenheit, enough to fracture clay tile and warp a metal liner.

Scraping glaze off clay tile tends to damage the tile, so the first step is usually a chemical modifier, applied by spray or over several burns. It dries the glaze into something brittle that rotary tools can break away. When glaze has soaked into cracked tile, removal stops being practical and the honest answer is a new liner.

How to tell which stage you have

Three simple observations with the fireplace cold will tell you which creosote stage is on your flue walls.

• Fingernail test: reach above the damper and scratch. Powder is stage 1. Crunchy flakes are stage 2. A surface your nail slides across is stage 3.
• Puffy, rainbow-tinged honeycomb means creosote has already burned once. You have had a chimney fire, possibly without noticing.
• Dark stains on the outside of the chimney or a sharp, acrid smell in damp weather point toward heavy deposits.

Why it matters around Hobart

Cold exterior chimneys and inserts venting into oversized flues are the two regional conditions that push creosote toward glaze. A masonry chimney on an outside wall takes a long time to warm when lake wind is pulling heat off the brick, and a cold flue condenses more tar. The region also has plenty of older homes with large clay-tiled fireplace flues, and many of those fireplaces now hold an insert whose small exhaust was never matched with a properly sized liner. If that describes your house, our Hobart chimney cleaning and inspection crew can put a camera in the flue and show you exactly what is there.

Keeping deposits at stage 1

Creosote stays at stage 1 when the wood is dry, the fire has air and the flue is warm. Burn split wood dried to under 20 percent moisture, the level the EPA’s Burn Wise program recommends. Give the fire enough air to show active yellow flame instead of a lazy orange glow. If you heat with an insert, have the venting checked against the appliance outlet size; our chimney relining page explains how liner sizing changes draft. And have the flue inspected yearly, because the stage can change in a single season of damp wood.