Why Does My Fireplace Smoke?


Smoke stains above the fireplace are proof of a smoking problem. Some detective work is sometimes needed to determine the cause of the problem before it can be fixed. Soot stains are CARBON particles. If you have soot stains inside your home, you have also had CARBON MONOXIDE fumes in your home. Soot stains are not only unsightly they indicate air in your home has been exposed to dangerous fumes. Take care of this problem (which may not be caused by your fireplace) before using it again. Soot and carbon monoxide are a natural byproduct of burning wood, gas, coal and oil - any carbon based fuel.

The first thing to check is that the damper has been opened before starting a fire!

Next, try priming the flue. Keep in mind that in the winter the chimney may be filled with cold air. Since warm air rises and cool air falls,you must reverse the air flow, sending warm air up the flue. You can do this by using a bit of newspaper, tightly rolled and lit like a torch and held up through the fireplace damper. Once the smoke from your newspaper torch reverses and sends the smoke up the chimney, continue with lighting the fire. Or, use a hair dryer to blow warm air up the chimney for a few minutes before lighting your fire.

Fireplace Size

Large fireplaces are a stunningly beautiful feature in any room. However, care must be taken in the planning and construction of a large fireplace in order for it to perform properly without causing a smoking problem.

Fireplaces must built within a general set of guidelines for proper sizing. Standard American fireplaces built with a sloped and angled rear wall to form a flattened smoke shelf should be designed with a 10:1 ratio of fireplace opening to flue size (square or rectangular flues). Length x width of fireplace opening provides the fireplace size length x width of flue tile provides the cross-sectional area of the chimney. Severe rectangles may need a 8:1 ratio. Round flues should provide a minimal 12:1 ratio as a round flue drafts more easily.

When the fireplace opening is overly large for the size of the flue, the chimney cannot adequately remove the byproducts of combustion the amount of air entering the fireplace must be commensurate to the amount of air that can exit the chimney. Since it's usually much easier to reduce the size of the fireplace opening than to enlarge the chimney, several options may be available:

Install a smoke guard, which is a metal bar that attaches to the fireplace opening at the top. Install glass doors the frame will overlap the opening on all 4 sides, thereby reducing the opening considerably. Raise the fire by laying a row of brick on the floor, using tall grates or tall andirons. Rebuild the firebox, which is a more expensive option. Install a fireplace insert (wood, gas, pellet, coal) with a new chimney liner sized appropriately for the fireplace.

Multi-Sided Fireplaces

Because of the explanation above, see-through or multi-side fireplaces almost always smoke because the flue must be commensurately larger to carry the extra volume of air needed by a multi-sided fireplace. Before building a multi-side fireplace, check that the chimney flue ratio has been planned carefully.

We're frequently asked about removing the rear wall of an existing fireplace to make it a see-through to the room behind it. A chimney that was designed to vent a fireplace with a single opening cannot handle having its fireplace opening size doubled and be able to work properly. Even if you find a brick mason who will do this job for you, it's not a good idea and almost never functions properly.

Chimney Design

An exterior chimney stays colder in the winter, especially in harsh climates. Before a chimney can draft properly it needs to be warmed, so an exterior chimney, although a common design, can present drafting problems. If you're building a home, consider a chimney built to the interior of the home if possible.

Chimney Height

The chimney needs to be at least 3' higher than the roof where it penetrates AND 2' taller than anything within 10' away. The shorter chimney at left is likely to smoke because of the taller chimney just a few feet away.

On chimneys with multiple flues, like the taller one in the picture shown, the flue tiles should be staggered in height to help prevent one flue from sucking smoke downward from the adjacent one. A chimney should be built so that its flues can be as straight as possible bends and offsets increase resistance and slowing the exit of the smoke and fumes which can cause drafting problems.

It's preferable to have the chimney built closer to the peak of the roof than on the lower side to reduce problems associated with a stack effect within the house, where there is a great difference in pressure between the air in the house and outdoor air. Air within the house leaves it, often from the upper sections such as the roof or upstairs windows. Entry air must be provided to replace the air that is exiting. When the chimney is on the lower side of the roof, replacement air may be drawn down the chimney. This can cause a competition between cold air being sucked down the chimney while the smoke is simultaneously trying to rise from it. A common problem is a chimney located in a one story room of a 2 story house which commonly suffers from negative pressure problems. Negative Pressure

In our compulsive obsession to design energy-efficient "air tight" homes, we often don't consider there must be means for outdoor air to enter the house to operate the devices we use in them. Chimneys must pull air from somewhere to provide combustion air for the fire AND allow an updraft so smoke and fumes can exit, so air must be supplied at an equivalent rate to replace the air leaving the chimney. Extremely airtight homes can prevent chimneys from operating properly, especially where other air-moving devices are being used such as furnaces, bathroom or kitchen vents, attic fans, clothes dryers, etc. Again, replacement air for these devices AND for the chimney may be entering through the chimney. It is possible to have cold air dropping down one side of flue while warm smoke or fumes are also trying to exit at the same time. Try cracking the closest window to the fireplace to provide extra air for the fireplace AND make sure no other air-moving devices are being used in your home at the same time.

If your chimney once worked well and now doesn't, consider whether you may have made alterations that affect its ability to draft, such as weatherstripping, replacement windows, new siding, extra insulation, room additions, or new appliances. High efficiency clothes dryers that dry clothes more quickly can use drastically more air to operate.

Chimney and fireplace professionals may be able to suggest alterations to improve or cure your smoking or malfunctioning fireplace in SOME situations. If your new stove or fireplace is not working properly, have it checked by a pro for advice. But remember, using the best principles of design for a fireplace or stove installation may not be overcome by the design of the house and the way that air enters and leaves it. Simply put, we in the fireplace industry cannot solve smoking or draft problems in every situation.

*Article reprinted with permission of Victorian Fireplace Shop

Karen Duke is a fireplace, chimney and hearth industry expert of over 25 years in both the retail and service sectors. She is a CSIA Certified Chimney Sweep and has numerous hearth industry certifications. She is the founder and webmaster of http://www.TheFireplaceChannel.com and she is the co-founder and webmaster ofhttp://www.TheVictorianFireplace.com, which is one of the largest online fireplace retailers in the world. She makes her home in Mechanicsville, Virginia. Karen's contact information can be found on either of the above sites.


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