Why smoke spills out of the fireplace
Smoke in the room is a draft problem. The causes run from a half-open damper to a house that pulls air down the flue, and two of them you can test yourself.
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In short
Fireplace smoke coming into the room means the chimney is not drafting strongly enough to pull the smoke up and out. Draft comes from warm flue gas rising and drawing room air in behind it. The causes include a cold flue, a closed or broken damper, a blocked flue, a house under negative pressure, a chimney too short for its roof, or an opening too large for the flue.
How chimney draft works
Hot gas from a fire is lighter than the cooler air around it, so it rises up the flue. As it rises, it lowers the pressure at the bottom of the chimney, and room air flows into the fireplace opening to replace it, carrying the smoke with it. That steady pull is draft. The warmer the gas inside the flue and the taller the chimney, the stronger the pull.
Chimney draft problems come from anything that weakens the rise or blocks the path, and they show up at the opening as smoke rolling out under the lintel. The fireplace also has to be in proportion to its flue. For a masonry fireplace, one of the Florida Building Code's sizing methods sets the flue area against the size of the opening, at no less than one-tenth of the opening for a square flue or a rectangular one less than twice as long as it is wide, and one-twelfth for a round one. A fireplace whose opening has been enlarged, or whose flue was undersized from the start, can smoke even when everything else is right.
The damper comes first
The simplest cause is a damper that is not fully open. Handles get installed backward, chains slip, and a plate can drop off its pivot and sit across the throat. With no fire burning, open the damper and aim a flashlight up past the lintel. An open damper shows the flue above it, and a closed or fallen plate shows metal.
A damper that will not open all the way, has rusted in place or has warped out of its frame needs attention before the next fire. Our guide to a fireplace damper stuck in one position walks through the checks.
A cold flue at lighting
A chimney not drawing in the first minutes, with smoke that spills and then clears as the fire gets going, points to a cold flue. Until the chimney warms, the column of cold air inside it can sink rather than rise, and it pushes the first smoke back into the room. Chimneys on an outside wall, and ones that have not been used for a while, show this most.
Warming the flue before lighting helps. Open the damper, light a twist of newspaper and hold it just below the damper until you feel the air begin to pull upward, then light the fire. Build it toward the back of the firebox with dry kindling so it catches quickly and gets hot fast. If smoke still spills after the fire is well established, the cause is something else.
Blockage in the flue or the cap
A flue narrowed by a nest, fallen leaves, broken tile or heavy creosote cannot carry the smoke a clear one would. The smoke shelf behind the damper collects debris that falls down the flue, and a pile there can choke the throat. A cap's spark screen can also clog with soot and creosote until it restricts the flue outlet.
A look up the flue with a flashlight shows the obvious cases, but much of a flue is out of sight from below. A sweep clears the flue, the shelf and the cap screen, and the note from the visit says what was blocking it. If animals were involved, a screened cap closes the path in.
A house that pulls air down the chimney
A house is a closed box, and every exhaust fan that pushes air out has to be matched by air coming in somewhere. When a kitchen hood, bath fans or a clothes dryer are running, the house can fall into negative pressure, and the easiest way in for replacement air is down the fireplace flue, against the smoke. Tighter houses show this more, since they have fewer small leaks to make up the air.
The Florida Building Code recognizes the problem. It asks for an exterior air supply to a fireplace unless the room is mechanically ventilated and controlled so the indoor pressure is neutral or positive. Test it yourself by cracking a window near the fireplace while the fire burns. If the smoke stops, pressure is the cause. The fix may be as simple as not running the hood during a fire, or it may be a makeup air solution from a heating and cooling contractor, which is outside our trade.
Chimney height, wind and nearby roofs
The Florida Building Code asks for a chimney to rise at least 3 feet above the roof at the highest spot where the two meet, and at least 2 feet taller than anything on the house within 10 feet of it. A chimney that falls short of that, or one sitting below a taller roof section, a second story or a large tree, can have wind tumbling over the nearby surface and pushing down into the flue.
Wind-related smoking comes and goes with the weather and the wind direction, which is a clue in itself. Some cap designs are made to resist downdrafts, and the right one depends on how the wind hits the top of the stack. Sometimes the lasting answer is more height, which is a larger project, and the written estimate names who would do that work.
An opening too big, or a rough smoke chamber
When the flue is too small for the opening, the fireplace can pull only so much air, and the rest of the smoke spills out at the top of the opening. A metal smoke guard fixed across the top of the opening reduces its height and can be enough to stop it. Building the fire toward the back of the firebox, under the throat, also helps the smoke find the flue.
The smoke chamber above the damper is meant to funnel smoke smoothly into the flue. A chamber with stepped, rough or crumbling walls slows the smoke and makes eddies that can push it back into the room. Parging the chamber smooth with a refractory mortar is a repair we check for when the damper, the flue and the house pressure all check out.
When smoke means stop now
Smoke coming out of the fireplace carries carbon monoxide and fine particles, so a smoky fireplace is not only a nuisance. When a carbon monoxide alarm sounds, or smoke seeps from walls, ceilings or a room upstairs, get everyone out and call 911. Smoke coming through masonry or into another room can mean a crack or open joint in the flue, which is a fire risk as well as a smoke one.
Otherwise, leave the fireplace unlit until the cause is found. The chimney's yearly inspection is due anyway, and a smoke problem is a good reason to book it early. We check the damper, the flue, the smoke chamber, the cap and the height, and run a draft test with the house operating as it normally does. The written estimate names the cause and the fix, and it says plainly when the fix belongs to another trade.
FAQ
Frequently asked questions
Why does my fireplace smoke only when the fire is first lit?
A cold flue: the cold air inside it sinks until the fire warms it. Warm the flue first with a rolled sheet of lit newspaper held near the open damper, then light a small, hot fire at the back of the firebox. If smoke keeps spilling after the fire is established, the cause is elsewhere.
Why does smoke come into the room when the kitchen hood is on?
The hood pushes air out of the house, and replacement air comes in by the easiest path, which can be straight down the chimney. Turn the hood off or crack a window near the fireplace while a fire burns. If that fixes it, the house needs a better source of makeup air.
Can a chimney cap make a fireplace smoke?
Yes, if its spark screen has clogged with soot or creosote and restricts the flue outlet, or if the cap is too small or the wrong style for the wind up there. Cleaning the screen or fitting a cap sized to the flue can fix it.
Can a smoke guard stop a fireplace from smoking?
It can, when the opening is too large for the flue. A smoke guard is a metal strip fixed across the top of the opening that lowers its height so the flue can keep up. It will not help a closed damper, a blocked flue or a house pulling air down the chimney, so those are ruled out first.
Is a smoky fireplace dangerous?
Yes, it can be. Smoke carries carbon monoxide and fine particles, and smoke seeping from walls or into other rooms can mean a crack in the flue. Hold off on fires until the cause is found, and if the CO alarm goes off, get everyone outside and dial 911.
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Guides
Chimney and fireplace guides
- Where chimney and fireplace odors come fromThe smell is air from the chimney arriving in the room. Find what that air passed over, and you have the fix.
- Blue, yellow, lazy or sooty: what a gas fireplace flame is telling youThe flames built into the design, and the ones that call for service.
- Why the pilot goes out, and what you can safely checkThe thermocouple, the flame that heats it, and the drafts and vents that interfere.
- When the gas fireplace will not turn onSafe checks in the right order, and the point where a homeowner stops.
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