Creosote, explained: how chimney fires actually start

What creosote is

Wood never burns completely. The unburned portion rises as vapor — tar droplets, water, and organic compounds — and when it touches the cooler walls of the flue, it condenses and sticks. That residue is creosote. It's chemically closer to a fuel than to ash: given enough heat, it ignites. The cooler the flue and the smokier the fire, the faster it accumulates.

The three stages

Sweeps grade creosote by how it has cured:

• Stage 1 — velvety soot. Loose and dusty, brushes off easily. What you get from hot, well-aired fires with dry wood.
• Stage 2 — crunchy flakes. Hardened, shiny black flakes like burnt cornflakes. Removable with rotary brushes; this is where most neglected chimneys sit.
• Stage 3 — glaze. A dense, tar-like coating that has melted and re-hardened, layer over layer. It's concentrated fuel, it's the hardest to remove, and it's the stage most chimney fires feed on.

The stages aren't just about difficulty — each one is more flammable than the last. That's why the standard is to clean at 1/8 inch of buildup, before anything approaches glaze.

The fire you hear — and the one you don't

A dramatic chimney fire announces itself: a low roar like a freight train, dense smoke, sometimes flames or sparks at the top of the stack. If that ever happens, get everyone outside and call 911 — then have the flue camera-inspected before it's used again.

But most chimney fires are slow, starved of air, and quiet. They smolder in the creosote, reach temperatures that crack clay tiles and scorch nearby framing, and burn out unnoticed. We regularly find the aftermath — puffy, honeycombed creosote and split tiles — in flues whose owners never suspected a thing. A damaged liner from a past fire is precisely what a Level 2 inspection exists to catch.

Three habits that prevent nearly all of it

• Burn dry, seasoned wood. Wood split and dried for 6–12 months burns hot and clean. Wet wood cools the fire, makes smoke, and smoke makes creosote. In foggy SF, store wood covered and off the ground.
• Burn hotter, shorter fires. A bright, well-aired fire deposits far less than a damped-down smolder. Resist the urge to choke the airflow for an all-nighter.
• Keep the annual appointment. An annual sweep and inspection resets the flue to zero. Everything above only slows creosote down; the brush is what removes it.

The bottom line

Creosote is a byproduct you can't avoid, only manage — and managed, it's a non-issue. If you can't remember your flue's last cleaning, that's the sign. Call (650) 722-5502 and we'll take a look, any hour.