Homeowners often ask what causes a fireplace to smoke during use. One answer to this question is that your home could be on fire. You must take all precaution that this is not the case. Many home fires are caused by the presence of combustible construction materials close to the firebox, smoke chamber, or flue of a fireplace. Unfortunately, many of our repair requests are after a fire has occurred because of this condition. It is also unfortunate that these problems cannot easily be detected except during construction or repairs. This is why it is important to always remember that lighting a fireplace does increase your risk and you should do so with all due caution and preparation for any problem which can occur.
Most of the smoking problems associated with fireplace use are the result of "negative pressure" occurrences. Negative pressure of a home occurs when the air pressure in the home is less than the air pressure outside the home. During negative pressure events outside air will enter your home through any opening it can find. Unfortunately, sometimes the pathway that is available is your chimney and its flues. There are several ways by which the air pressure in your home can be lowered. Both mechanical and natural conditions can produce negative pressure in your home. Often, we find that a home built or remodeled in the last ten to fifteen years is of such tight construction that little outside air infiltration occurs. In these homes both natural and mechanical negative pressurization can easily cause a fireplace to smoke. It will help you to consider the following examples of negative pressurization to see if they might apply to your situation.
Forced draft furnaces
These four appliances cause many of the negative pressure situations in your home caused by mechanical means. They literally suck air out of the home and put it outside. Often the fireplace flue is the only opening available for the home to get air while one or several of such appliances are operating. The result of air being sucked down the flue causes smoke to come into the home. You can reduce the occurrence of such negative pressure by supplying makeup air to the fireplace or home. Sometimes something as simple as cracking a window or sliding door will overcome the negative pressure and cure your smoking fireplace. When we discuss natural causes of negative pressure, we will have some reminders as to the importance of which openings should be used to reduce negative pressure. Sometimes a special (combustion air) vent will need to be installed into the firebox of the fireplace to supply combustion air for the fire.
Yes, the fireplace itself can cause a negative pressure situation which can only be relieved by bringing air (and smoke) back down the fireplace flue. This occurs because the process of combustion (fire) uses oxygen (and air). A tight home can literally use up enough air that the fireplace flue is the only route for a fresh supply of outside air available. This causes smoke to enter the home. Often this occurrence is characterized by a "burp" of smoke coming out of the fireplace. This can occur many times but is usually separated by a period of non smoking.
Any flue needs to be sound and without air leaks to carry out its venting processes efficiently. If the flue is cracked, has missing sealant between the flues, or is unsound in any manor it's ability to work can be greatly compromised. These problems can be caused by a chimney fire, the deterioration of the flue lining material, or construction deficiencies.
The majority of the masonry flues we use today are either rectangular or square in shape. These flues need to be oversized because of the inherently poor venting characteristics of these shapes. A round flue (such as SOLID/FLUE) which is insulated and one piece from top to bottom can be sized considerably different than the rectangular or square flues. The efficiency gain of the SOLID/FLUE insulated cast refractory flue can be over twenty percent above the rectangular or square flue.
The air pressure on the side of your home which is receiving a wind will have a higher pressure than the side away from the wind direction. Because the air pressure on the downwind side of your home can be lower if you should be trying to supply make up air by opening a door or window on the side of the house away from the wind you will actually increase the negative pressure of your home. A make up air supply should always be on the side of the home which faces the wind. This will pressurize the home and should make smoke go up the chimney much more easily.
Trees and high structures
Again if high trees are up wind of your chimney, higher pressure can be caused at the top of the chimney by the downward curl of the wind going over the tree or adjacent structure (higher roof). These situations can cause your fireplace to smoke. It is also possible to have a lower pressure caused by wind and structure if the wind path should pull a void (vacuum) over your chimney. This occurrence might help your chimney to vent cleanly. As you can see several items could be singularly or in combination cause a fireplace to not work properly. We hope that by considering these possibilities that you and we can solve your operational problems.
We suggest that if you don't have a good set of glass doors and screen on your fireplace that you consider a set and then consider asking a professional for any ongoing problems. This information has been presented by Donald Fuller of Central Michigan Chimney and should be only considered as the most basic of primer for the correction of fireplace smoking problems.
This helpful article was provided by DoItYourself.com community member Don Fuller.