WHY DOES MY FIREPLACE SMOKE?
Smoke stains on the outside of this fireplace are proof of a smoking problem. This is a primary safety concern because soot stains are CARBON particles. If you have soot stains inside your home, you have also had CARBON MONOXIDE fumes in your home. Simiply put, soot stains are indicative of a home that has been exposed to dangerous fumes. Be sure to remedy the smoking problem before further use of your fireplace! Soot and carbon monoxide are a natural byproduct of burning carbon based fuels: wood, gas, kerosene,coal and oil.
Sometimes the remedy is simple. The first thing to check is that you've opened the damper before starting a fire!
Next, try priming the flue. In the winter the chimney may be filled with cold air, especially if it's an exterior chimney (built on the side of your house vs. in the middle of your home). Since warm air rises and cool air falls,you must reverse the air flow, sending warm air up the flue. Tightly roll some newspaper, light the end and hold it, torch- style, through the open 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 but they're somewhat notorious for having smoking problems due to improper design of the firebox, smoke chamber and/or their chimneys. 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 volume of combustion 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 insert. This option usually provides you considerably more heat from less fuel as well as solving your smoky fireplace problem.
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. If you're lucky enough to be reading this BEFORE you builid 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 never functions properly.
CHIMNEY DESIGN An exterior chimney stays colder in the winter, especially in colder 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. Here are other common chimney conditions to check if your fireplace smokes:
Chimney Height. The chimney needs to be at least 3' higher than the roof where it penetrates AND 2' taller than anything (roof, trees, etc.) within 10' 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, 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 addition of a 2 story house which commonly suffers from negative pressure problems, such as the home pictured at right.
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 that house. Chimneys must pull air into the house 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. (Picture sucking through a straw while blocking the other end of the straw with your finger... no air moves through!) 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 downwards through the chimney flue. It is possible to have cold air dropping down one side of the 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 you suddenly suffer from smoky odors from your fireplace, have a chimney sweep check for bird or animal nests that may be blocking the chimney. Also 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. Newer, high efficiency clothes dryers that dry clothes more quickly can require drastically more air intake to operate properly.
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 might not be able to 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. In these cases we recommend you install an insert of some type as the most practical solution.
There is a gas appliance category called direct vent that offers a balanced flue design that is not affected by tight homes. This appliance will solve drafting problems nearly 100% of the time; I've personally never encountered a home with problematic drafting that prevented a direct vent from working properly.
This article reprinted courtesy of Victorian Fireplace Shop.