· by Michael Blair, Nashville's Chimney Sweep
A wood fire in an open fireplace depends on the chimney drawing air up and out faster than the fire makes smoke. The Department of Energy’s Building America Solution Center describes most fireplaces in existing homes as natural-draft appliances that take their air from the room and rely on a heated chimney to pull the smoke up and out of the flue. When that pull fails, the smoke comes out into the room instead. Some causes are about how the fire was started and are fixed in a minute. Some are about the house and the flue and are fixed with a part or a change in habit. And a few are about the chimney itself and need someone to look inside it. The pattern of when it smokes tells you which.
Only at the start: a cold flue
A chimney on an exterior wall in January is full of cold air, and a cold flue draws poorly. University of Missouri Extension’s guide to chimneys, written for wood stoves, explains that a warm chimney gives more draft than a cool one, which is why a stove works better once it has warmed the flue than when it is first lit, and why a chimney inside the house drafts better than one built entirely outside it. Until the flue warms, the smoke takes the easier path into the room. It is a common cause, and the sign is a fireplace that smokes at the start and then draws fine once the fire is going.
Warming the flue first is the fix. What we tell people is to open the damper, hold a rolled, lit newspaper up near it until they feel the air start to pull upward, and only then light the fire. The kindling matters too: the EPA’s wood-burning guidance says to start fires with newspaper, dry kindling or natural fire starters, and Missouri Extension adds that green or wet wood, or too cool a fire, can keep a chimney from warming up and developing full draft. Our post on firewood that will not wreck your chimney covers what seasoned wood means and how to keep it dry.
Why does a fireplace smoke whenever a fan is running?
A chimney needs replacement air. When the area around a fireplace is depressurized relative to the flue, the Building America Solution Center says, combustion gases spill into the house rather than going up the flue, and a fireplace with a cool chimney can be backdrafted by a kitchen or bath exhaust fan of as little as 50 CFM. A range hood, a dryer or a bathroom fan running while the fire burns is enough to turn the chimney into an air intake. Tightly built and air-sealed houses are prone to it, and Missouri Extension names a new, tightly constructed home as one where there may not be enough openings to supply the fire. The tell is a fire that smokes whenever the kitchen fan is on, or that draws better with a window cracked.
Cracking a window in the same room takes the pressure off, which is the fix Missouri Extension suggests. The permanent answer is a dedicated combustion air duct that brings outside air straight into the firebox, which the Department of Energy’s guidance on fireplace ventilation recommends for a natural-draft fireplace, and the Solution Center pairs it with tight-fitting glass doors across the opening. The residential code asks the same of new fireplaces: the chapter on chimneys and fireplaces, as published in Ohio’s administrative code, requires an exterior air supply unless the room is mechanically ventilated to a neutral or positive pressure. Tennessee’s State Fire Marshal’s Office lists the 2018 International Residential Code, with amendments, among the codes the state has adopted. Older fireplaces may have nothing of the kind, and an inspection tells you whether yours does.
- Smokes at the start, then fine: cold flue, warm it first
- Smokes when a fan runs or in a tight house: makeup air
- Smokes in some winds only: cap, chimney height, a nearby roof or tall trees
- Smokes always, regardless: flue size, a blockage or a damper that will not open
How big should a fireplace flue be?
An open fireplace needs a flue sized to its opening. The same code chapter sets the minimums: a round flue needs a net area of at least one-twelfth of the fireplace opening, a square flue one-tenth, and a long rectangular flue, twice as wide as it is deep or more, one-eighth. A fireplace built or remodeled with an oversized opening can fall below that, and so can a chimney relined with a liner smaller than the original flue. The result is a fireplace that never draws well no matter how the fire is built, which is why the opening is worth measuring before a chimney liner is sized.
The fixes are to shrink the opening or to strengthen the draft. A smoke guard, a metal strip across the top of the opening, lowers the opening and cuts its area, so the flue has less to handle; how much it helps depends on how far out of proportion the two are. For draft, Missouri Extension notes that the taller the chimney, the greater the draft, and that a hood or cap can sometimes keep wind eddies from interfering with it. Those are the bigger jobs, and the measurements say whether they are needed.
Why does a fireplace start smoking after years of drawing well?
A fireplace that drew fine last year and smokes this year points to something that changed, and an obstruction is the first thing we look for. Missouri Extension lists bird nests and fallen bricks among the things found in chimneys and says any object that restricts the flow of exhaust can cause smoky fires; our post on birds, squirrels and bats covers the nesting side. We also check that the cap is clear and that the damper opens all the way. A chimney that has never drawn well may be too short for the roof or the trees around it. The same guide says the eddies that upset draft most often come from wind blowing across nearby parts of the roof, that tall trees nearby can make enough clearance impossible, and that an unusual wind direction can cause occasional smoking. The code’s floor is a chimney at least 3 feet above where it passes through the roof and 2 feet higher than any part of the building within 10 feet. Smoke that comes in only on windy days points that way.
These are what our chimney inspection is for: we run a camera up the flue and look at the cap, the damper and the liner. A chimney that smokes and has not been inspected in years should be looked at before the next fire, because what restricts the flue can include creosote. A U.S. Fire Administration report on residential heating fires, drawn from late-1990s data, found creosote to be the leading material ignited in chimney fires, and our post on creosote explains how it builds up. If yours smokes and you cannot tell which pattern it fits, ask us for a free estimate.
Sources
- U.S. Department of Energy, Building America Solution Center (PNNL), Pre-Retrofit Assessment of Combustion Appliances (2017) — Fireplaces as natural-draft appliances relying on the stack effect; depressurization and backdrafting; a cool chimney backdrafted by a 50 CFM kitchen or bath fan; outside air duct and tight-fitting glass doors
- University of Missouri Extension, G1732 Chimneys for Wood Stoves — Warm chimneys draft better; interior versus exterior chimneys; wet wood and cool fires; tight homes and opening a window; wind eddies, trees, height and caps; nests and fallen bricks
- EPA Burn Wise, Best Wood-Burning Practices — Start fires with newspaper, dry kindling or natural fire starters
- U.S. Department of Energy, Building Science Education Solution Center, HVAC Proper Ventilation for Fireplaces and Stoves — Dedicated combustion air duct bringing outside air into the firebox of a natural-draft fireplace
- Ohio Administrative Code 4101:8-10-01, Chimneys and fireplaces (Legal Information Institute) — Residential code text: exterior air supply (1006.1), flue-to-opening ratios (1003.15.1), chimney height above the roof (1003.9)
- Tennessee State Fire Marshal’s Office, Currently Adopted Codes — Tennessee adopts the 2018 International Residential Code with amendments
- U.S. Fire Administration, Heating Fires in Residential Structures (Topical Fire Research Series, 1996-98 data) — Creosote is the leading material ignited in chimney fires
FAQ
Why Smoke Comes Into the Room When You Light a Fire — questions
Why does my fireplace smoke when I first light it?
The flue is full of cool air and the draft has not built up yet. A warm chimney draws better than a cool one, so a fireplace that smokes for the first stretch and then draws well is showing a cold flue rather than a fault in the chimney. Open the damper, hold a lit, rolled newspaper up near it until you feel the air pull upward, then light the fire with dry kindling.
Why does the fireplace smoke when the kitchen fan is on?
The fan lowers the pressure in the house and the chimney becomes a path for replacement air, which pushes the smoke back down. A flue that is still cool, early in a fire, is the most easily overpowered, and even an ordinary kitchen or bath fan can do it. Turn the fan off, crack a window in the room, or have an outside air supply ducted to the firebox. Tight or well-sealed houses are especially prone to it.
Can a smoke guard fix a smoking fireplace?
It can, when the cause is an opening too large for the flue. The guard lowers the top of the opening and reduces its area, so the flue has less to carry. Whether that is enough depends on how far the opening is out of proportion with the flue, so both are worth measuring before buying one, and a blocked flue or a running exhaust fan will not be fixed by it.
Why is my wood stove smoking into the room?
Usually for the same reasons an open fireplace does. A cold flue draws poorly until it warms, and green or wet wood or too cool a fire can keep it from warming fully. Exhaust fans in a tight house, a nest or fallen brick in the flue, or wind eddies around a short chimney can do it too.