I’m sure that most folks have never stopped to consider the physics of their chimney, nor have they wanted to, but if you are experiencing problems the way your chimney naturally works (or should) is important to understand. Science explains why a properly constructed chimney works as it does to vent the smoke out of the home. Your chimney sweep can help you to understand these concepts if you become confused or think that your chimney may have a problem.
The air inside your home is constantly moving due to various factors, but mostly due to the fact that air is always trying to escape out of the home through cracks in the upper areas. As air exits the home, air from the outside flows in through cracks in the lower part of the home to replace what has been lost. How the air flows in and out of a particular home is influenced by various factors as well. The stack effect, which is a term for the process of hot air rising to the top portion of the home, is one of these variables. This air creates pressure as it builds up in the top of the home trying to escape. Replacement air is entering the home underneath this phenomenon adding more pressure to this air such that escapes through tiny openings in light fixtures, or around windows or doors.
In your home, between the area of higher pressure (the upper portion of the home) and the area of lower pressure (the lower portion of the home) is located a neutral pressure plane at which pressure is neutral (air is not trying to exit or enter the home). This fact may seem to be pure common sense, that a medium would be reached at some point, and that above and below would be pressurized accordingly. This pressure plane, however, directly affects the way that your fireplace works. When your fireplace is located below the plane, air will have the natural tendency to flow into the home via the chimney to become replacement air.
Another variable that affects the way that your chimney works is known as wind loading, which can be defined as the pressure caused
by wind on a home. Wind hitting one side of the building creates a higher pressure on that side and a lower pressure on the other. If your chimney works poorly when it is very windy out, opening windows or doors on the side of the house which is being struck by the wind will allow air to enter the home and help the chimney to draft.
Appliances inside the home like kitchen or bathroom fans and dryers can also remove air from the home, depressurizing it. These appliances often remove a lot of air from the home at a given time. Due to the sudden loss of air, the chimney will often experience drafting problems. Fireplaces and the like require a lot of air in order to fully combust the wood or other fuel they are burning, and if there is not sufficient air inside the home to use for combustion air will be drawn from outside to replace the air that is not inside.