Why Does a Chimney Release Smoke?

Ventilation

• When a chimney is working properly, it will vent smoke outdoors. The purpose of the chimney, including its outer masonry layer and its internal, lined layer, is to provide ventilation for the byproducts of burning wood, oil or gas. Depending on the type of fuel you use, you should be able to observe a reasonably consistent amount of smoke escaping from the top of your chimney and drifting away.

Downdrafting

• If your chimney releases smoke at its base instead of its top, allowing smoke to enter your home where the chimney meets a stove or through a fireplace, it may be due to downdrafting. Downdrafts occur when high winds alter the air pressure in your chimney, causing a downward flow of air that is more powerful than the hot smoke's tendency to rise. A chimney cap or exhaust fan can prevent downdrafts by blocking the effects of wind or providing additional upward force, respectively.

Chimney Fires

• Chimney fires occur when the soot that lines the inside of a chimney over time catches fire due to the hot air rising past it. A chimney fire may result in unusually thick, dark smoke coming out of the top of your chimney. It can also cause smoke to seep through the chimney's interior brickwork, filling your home or attic with smoke. In some cases you may be able to see flames coming from the top of the chimney, or you may hear them from inside your home. If you notice smoke that seems to indicate a chimney fire, call your local fire department immediately.

Clogging

• Like other types of vents, chimneys can become clogged. This may cause them to emit smoke inside your home, either through cracks in the brick or out through your fireplace or stove. Clogging can come from snow buildup on your roof, debris inside the chimney or something covering the chimney's cap, such as a bird's nest. To prevent this type of smoking, clean out your chimney and ensure that its cap rises far enough above your roof to prevent snow and ice from covering it. Install a mesh cover to keep out animals and protect the chimney from collecting debris.