Why Is My Chimney Not Drawing Smoke Up?

How draft works

Picture the flue as a column of warm gas standing inside a column of cold outdoor air. The warm column is lighter, so it rises, and as it rises it pulls air from the room into the firebox to replace it. The hotter the flue and the taller the column, the stronger that pull. Trades call it draft, and in an open fireplace it is what carries every bit of smoke out of the house.

Anything that cools the column, shortens it, chokes it or lets cold air into its middle weakens the draft. When the pull falls below what the fire produces, smoke spills from the upper edge of the fireplace opening, usually right under the lintel, and into the room.

A simple draft test

With no fire and the damper fully open, light an incense stick or blow out a match and hold it just inside the fireplace opening, up near the damper. If the smoke streams upward into the flue, the chimney is drawing. If it drifts into the room, air is moving down the chimney, and a fire lit now will smoke.

Then repeat it with every exhaust fan, the range hood and the dryer off, and a window near the fireplace cracked open. Smoke that now goes up means the chimney is fine and the house is the problem. Smoke that still comes down points to the chimney itself, its height or something inside the flue.

The usual causes, most common first

In older brick houses, from city rowhouses to suburban colonials, weak draft tends to come from the same short list. Several can act at once, which is why a fireplace that drew well for years can start smoking after a renovation or a new boiler.

• A cold flue at startup, worst in chimneys on an outside wall
• A house short of air because of fans, a dryer, a boiler or tight new windows
• Partial blockage: nests, leaves, fallen tile or heavy creosote
• A flue too large or too small for the fireplace opening
• A damper that does not open fully or has rusted out of shape
• Cold air leaking in through cracked tiles, gaps between flues or an open cleanout
• A chimney too short for the roof or for a taller building next door

A cold flue at startup

A masonry chimney on an exterior wall can be colder inside than the room it serves. Cold air in the flue is heavy, and it sits there like a plug, so the first smoke from a new fire rolls back out instead of rising. Once the flue warms, the fire usually draws normally, which is the giveaway: it smokes for the first few minutes and then settles.

The common remedy is to warm the flue before lighting. Hold a rolled sheet of newspaper, lit at one end, up near the open damper for a moment until you feel the air start to move upward, then light the fire. Building the fire with kindling on top of the larger pieces helps too, because it produces a burst of heat quickly. If the problem persists after that, the cause is elsewhere.

A house that is short of air

Every cubic foot of smoke that goes up the chimney has to be replaced by air coming into the house. When exhaust fans, a clothes dryer or a boiler are already pulling air out, and the windows and doors are tight, the chimney becomes the easiest way in. Air then comes down the flue and pushes against the fire.

Tall, narrow houses add a second effect. In a brownstone or rowhouse, warm air rising and leaking out of the upper floors pulls air in at the lower ones, which is exactly where the parlor fireplace sits. The fixes are about giving the house air: crack a window near the fireplace, run fewer fans during a fire, or add a proper outside air supply.

Height and the roof around the chimney

A short chimney has a short column of warm gas and less pull. It also sits in the turbulent air that swirls over a roof. The widely used rule for masonry chimneys is that the top should be at least 3 feet above the roof where it passes through and at least 2 feet higher than any part of the roof or building within 10 feet.

Chimneys that met that rule when built can end up too low after a neighbor adds a story or a rooftop bulkhead. The answer then is an extension of the chimney, or a cap designed for wind, chosen after looking at how the air moves around that particular roof.

Problems inside the flue and damper

A flue that is too large for the fireplace stays cool and draws weakly; one that is too small cannot carry all the smoke. Cracked clay tiles, gaps between neighboring flues, or an open or rusted cleanout door let cold air leak into the flue and break up the column. Creosote, nests and debris on the smoke shelf narrow it.

The damper is a frequent culprit in old fireplaces. A cast iron plate that has warped, or a handle that slips so the plate opens only part way, cuts the throat opening without anyone noticing. A rough smoke chamber with stepped brick inside it also creates turbulence right where the smoke needs to accelerate.

When weak draft is urgent

A fireplace that smokes is unpleasant. A heating flue that does not draw is dangerous, because boiler, furnace and water heater exhaust carries carbon monoxide, a gas with no color and no odor. Soot around the draft hood of a water heater, a hot or damp feel at the flue's base, or a carbon monoxide alarm are warning signs.

When a carbon monoxide alarm goes off, or someone in the house starts to feel ill, leave the building first and phone 911 or the gas company once outside. Working alarms belong on each floor and close to the bedrooms.

What an inspection looks for

Finding the cause means looking at the whole system: the firebox opening and damper, the smoke chamber, the full length of the flue from top and bottom, the cleanout, the cap and the chimney's height compared with the roof and nearby buildings. The technician also asks what runs in the house during a fire, because fans and appliances are part of the story.

You get a plain explanation of what was found and, where a repair is needed, a written estimate before anything is done. Many draft problems turn out to be simple, like a damper that does not open fully or a nest at the top.