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Q: I was alarmed to see smoke enter the living room when I last used my fireplace. Why was my fireplace smoking, and how can I keep it from happening again?
A: Whether it’s the first or last burn of the season, you should never notice smoke in your home after starting a fire. When your fireplace and chimney vent as they should, fire by-products (such as smoke, vapor, and unburned wood) are pushed up the flue (the space inside the chimney) and out of the house while the outside air is pulled into the flue to keep the flames alive. This vital exchange of air is known as the chimney “draft.”
A fireplace that kicks up smoke is a classic sign of a weak draft, which can result in a fire that quickly dies out or fire by-products “back-puffing”—getting backed up in the firebox or flue and issuing into the room as smoke and harmful vapors, including carbon monoxide. A draft problem can have many causes. The main causes are explored below, along with tips for spotting and solving each one so you can breathe easier and enjoy your fireplace.
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If the indoor-outdoor temperature differential is too low, light your fires when it’s colder outside.
The strength of the chimney draft depends on the difference between the indoor and outdoor temperatures. The greater the temperature difference, the stronger the draft; the lower the difference, the weaker the draft.
Thus, when it’s cold outside and warm inside, hot air and fire by-products will rise up the flue to meet the cold air outdoors. When it’s about as warm outside as it is indoors, hot air and fire by-products will float in the firebox or enter the room rather than rise up and out of the chimney. Similarly, proper drafting won’t occur when the flue is cold, as hot air will float in the cold flue rather than rise to the top of the chimney.
If your fireplace only seems to smoke when it’s warm outside, then a low indoor-outdoor temperature differential is likely to blame. To improve drafting, check your thermostat and the weather forecast and start a fire only when the difference between the indoor and outdoor temperature is at least 20 degrees.
Likewise, before starting a fire on below-freezing days or after the fireplace has been inactive for several months, light a rolled-up newspaper and hold it in the flue near the damper (just above the firebox) for one to two minutes to avoid a cold flue. Pre-warming the flue will increase the temperature differential between the flue and the outdoors, improving the draft and reducing smoke in the home.
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If water is seeping into the flue, have a chimney cap installed.
Rain or snow can easily seep into an uncovered flue. Once there, the water will lower the temperature of the air in the firebox and impede its ability to rise, weakening the draft to potentially cause back-puffing.
If the fireplace seems to kick back smoke only when or after it rains or snows, you may have a water seepage issue. To avoid a wet flue, have a chimney sweep certified by the Chimney Safety Institute of America (CSIA) install a chimney cap.
This covering installed around the outside opening of the flue keeps out water (along with animal nests and debris) to help strengthen the chimney draft and prevent indoor smoke.
If the flue is blocked, enlist a chimney inspector to remove the obstruction.
A flue can become blocked by leaf debris, animal nests, or the buildup of creosote, a dark brown to a black coating that forms in the chimney when fire by-products harden. These obstructions can reduce or prevent the passage of smoke from the firebox to the outdoors through the flue and lead to back-puffing.
Far more seriously, when the temperature in the flue is high enough, creosote build-up or debris can ignite a chimney fire that can do serious structural damage to your home.
Uncovering dark buildup when you scratch a finger against your chimney walls is a telltale sign of creosote buildup while observing nests or debris when you put a flashlight up your chimney flue can tip you off to a larger obstruction in the flue. If you spot either, call a CSIA-certified chimney sweep to inspect it and, if needed, clean the chimney to remove creosote buildup, nests, and other debris and keep back-puffing at bay.
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If your home has negative air pressure, have an outside air supply vent installed.
A strong chimney draft requires neutral air pressure—that is, outside air enters the home at the same rate that inside air exits it, so that indoor and outdoor air pressure is the same. This allows fire by-products to exit the flue as the outside air enters it.
However, in a house with negative air pressure—usually newer, energy-efficient homes that are well-sealed with weather stripping or caulking—more air enters the home than exits it, so outside air pressure is higher than that indoors. The greater influx of air from the outdoors pushes down smoke in the flue until it enters your home.
To determine if this is the case in your house, next time the fireplace is smoky, open a nearby window or door while the fireplace is in operation. If this seems to reduce or eliminate indoor smoke, your home probably has negative air pressure.
To address an issue of negative air pressure, consider having a mason install an air supply vent at the back of the firebox. This rectangular grate supplies air from the outdoors to the fire, balancing indoor-outdoor air pressure and encouraging fire by-products to escape the flue.
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If your chimney or fireplace has a design flaw, have a smoke guard installed.
If none of the above problems are behind your smoky fireplace, the culprit may be the chimney or fireplace itself. Proper drafting requires chimney and fireplace components to be built in a certain size. Examples include a flue that’s too small, a chimney that’s too short, or a lintel (horizontal support above the firebox opening) that’s too high—any of these could result in a weak draft and a smoky fireplace.
While having these structural components altered is often cost-prohibitive, a workaround is to install a smoke guard, such as the HY-C Smoke Guard, in front of the fireplace. This bar at the top of the fireplace opening limits the fire by-products that enter the home, minimizing your exposure to smoke.
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Follow best practices for smoke-free fireplace operation.
Stave off a smoky fireplace the next time you light up by following these fireplace operation and chimney maintenance tips:
- Use safe fuel, kindling, and tinder. Use only well-seasoned hardwood or CSIA-approved logs as fuel; dried twigs or branches as kindling; and torn old newspaper or pine cones as tinder. Burning unseasoned firewood or cardboard can generate an excessive amount of smoke that your chimney can’t efficiently evacuate.
- Employ the top-down burn method. That is, place the large logs vertically in the firebox, add four to five horizontal layers of kindling, top with tinder, and then light. This method for lighting a fireplace creates a hot, fast-burning fire, which minimizes smoke and vapor.
- Position the grate in the firebox so that there are at least a few inches around it on all sides. Fireplaces tend to produce more smoke when the fireplace grate is placed too close to the front of the firebox.
- Remove ashes from the firebox after use. When the firebox is completely cool, shovel the remaining ashes into a metal container. Ashes in the firebox from the last burn can cause the fireplace to emit more smoke.
- Have a CSIA-certified chimney sweep inspect your chimney annually. This bit of professional maintenance helps keep your chimney clean and free of obstructions or structural damage.