How Do You Know If Your Chimney Is on Fire?

What is actually burning

A chimney fire is not a big fire in the fireplace. It is the deposits on the inside walls of the flue catching fire, somewhere above the firebox. Those deposits are creosote: tar-like residue left when wood smoke cools on its way up. Sweeps describe three stages, from dry, flaky soot, through crunchy, tar-like flakes, to a hard, shiny glaze. All three burn, and the glazed stage burns the hottest and longest.

The spark is usually ordinary: a hot fire on a cold night, an armful of paper or a dry Christmas tree branch, or a stove opened up after a long, smoldering burn. Once the creosote catches, the fire feeds on the air rushing in through the fireplace or stove, and it can spread up the whole height of the flue within minutes.

Why some chimneys are more likely to catch

Creosote builds fastest where smoke cools fastest, and older houses have plenty of those places. Chimneys on outside walls lose heat to the weather all winter. Fireplaces built for coal often have flues larger than a wood fire can keep warm, so smoke crawls up slowly and condenses on the way. Unlined flues and cracked tiles give creosote rough surfaces to cling to and let heat escape into the brick.

Habits add to it. Damp wood, fires choked down with a half-closed damper, wood stoves turned low for an overnight burn and years without a sweep all build the deposits a chimney fire needs. None of these makes a fire certain, but each one makes the fuel load in the flue heavier.

Signs while it is happening

A fast, free-burning chimney fire is hard to miss. The fire pulls air through the fireplace so hard that the flue itself becomes loud:

• A deep roaring or rumbling, often compared to a train or a jet passing low
• Sharp popping, cracking or banging from inside the chimney or nearby walls
• Thick, dark smoke pouring from the top, or suddenly pushing into the room
• Sparks, burning flakes or flames shooting out of the chimney top
• A strong, hot, tar-like smell through the house
• A chimney breast, wall or ceiling near the chimney that feels hot

What to do first

Get everyone out of the house and call 911 from outside. Tell the dispatcher you think it is a chimney fire, and say if the house is attached or if a boiler shares the chimney. Do not go on the roof, and do not pour water down the flue: water hitting hot tile and brick can crack them, and the steam can burn you.

If you can do it in passing on the way out, close the glass doors of the fireplace or shut the door and air controls of a stove. Cutting the air supply slows the fire. Then leave, and let the fire department take it from there.

The quiet chimney fire

Some chimney fires make almost no sound. With limited air or a thin layer of fuel, the creosote smolders and burns slowly, sometimes for a long time, while the family sits in the next room. The temperatures inside the flue can still be high enough to crack clay tiles, open mortar joints and warp metal, and nobody finds out until the next sweep or inspection.

Slow fires are one reason a chimney can look fine from the living room and still be unsafe. The clues they leave behind are physical, and a sweep looks for them on every visit.

Some people do notice something at the time without recognizing it: a ticking or crackling from the chimney breast after the fire seems to have settled, a hot, sharp smell different from ordinary wood smoke, a wall beside the fireplace that feels warmer than usual, or crunchy black flakes on the roof, the car or the snow the next morning.

Clues that a fire already happened

If you have bought a house with a fireplace, or your chimney has not been checked in a while, these are the signs that point to an earlier chimney fire:

Stoves, inserts and heating appliance flues

A wood stove or fireplace insert can have a fire inside its stovepipe or liner. Signs include a stovepipe glowing dull red, a roar from the pipe, and smoke leaking where the pipe enters the chimney at the thimble. Stove chimney fires often follow long stretches of slow, low-air burning that coat the liner with creosote.

Oil boilers and furnaces do not make creosote, but they can leave heavy soot in a neglected flue, and soot can burn too. In brownstones and older homes where the boiler vents into a masonry chimney, a flue that has not been cleaned in years is a risk worth taking seriously.

Attached houses and shared chimney walls

In rowhouses and twins, a chimney often sits in the party wall, with your flue a single brick away from your neighbor's. A chimney fire in one flue heats the masonry shared by both, and cracks in old mortar can let smoke or heat reach the house next door. Knock on the neighbors' doors on your way out, and tell the fire department the houses are attached.

After the fire, the inspection should take in the whole chimney mass where it can, not only your own flue. Damage in a shared stack affects everyone who uses it.

After the fire is out

Do not use the fireplace, stove or heating appliance on that flue until the chimney has been inspected. Under NFPA 211, a chimney fire is one of the events that call for an inspection. A chimney that survived one fire may have cracked tiles, gaps in the mortar or a damaged liner, and the next fire could reach the framing around it.

We check the whole flue, explain what the fire did, and give you a written estimate for whatever has to be repaired or relined before the chimney goes back into service.