As some may know, chimneys are designed to, when a fire is lit and the flue system is warmed by the fire, to pull the byproducts of
combustion (smoke, toxic gases, etc.) up and out of the chimney by a natural draft that forms. As this warm air rises up and out of the chimney, air from other parts of the home move in to feed the fire, is heated, and rises up through the chimney as well. This cycle is ideal although is not always what comes to be.
You may have heard of negative pressure, which occurs when there is not enough air in the home to continue to feed the fire and keep the natural cycle of cold air warming and rising up and out of the chimney going. When there is not enough air in the home (say the home is too air tight or a kitchen or bathroom fan is removing air from the home) then air is actually pulled down the chimney to fuel the fire. This negative pressure causes smoke and other byproducts of combustion to come into your home instead of being flushed up and out of the chimney by new air.
Another problem that some homeowners may not be aware of, especially if you only get smoking problems with your chimney on occasion, is the role that wind plays in the proper draft of a chimney. On very windy days, wind may blow at an angle or the shape of your chimney or chimney cap may be just so with the angle of the wind to permit it to blow down the chimney. When these conditions exist, negative pressure is taken to a new level and can be considered adverse pressure; the pressure is actually moving against the chimney’s attempt at a natural cycle as opposed to being drawn into the cycle due to a lack of other air, which occurs in negative pressure problems.
If you are having a smoking problem, even if only on occasion, and especially if you have had your chimney looked at by a professional who could not seem to pinpoint the problem, you may want to consider keeping a journal on when you experience smoking problems and what the conditions are outside. Very cold days may allow the frigid air outside to drop down into the chimney if the flue is not warm enough because of the principle that cold air falls. Similarly, if you check the weather channel you often can get a reading of how hard the wind is blowing in your area and may be able to aid your professional in pinpointing a problem regarding a smoking fireplace and create a better wood burning experience for yourself and your family.