Chimney Basics
A chimney is a hollow structure which is designed to vent exhaust gases from an appliance — such as a fireplace, a wood or gas stove, a boiler, or a hot water heater — safely out of a building, steam locomotive, or ship and into the atmosphere.
Chimneys are generally as vertical as possible to help encourage the flow of air through them. They work through a mechanism known as the stack effect, wherein warm indoor air rises to the top of the house, causing cooler air to be drawn into the bottom of the house while the warm air passes out the top. A chimney intensifies the stack effect, allowing an active flow of air to carry gases away from the inside of the house and out through the top of the chimney.
The passage of air through a chimney is called draft (or draw). Factors that affect draft include the height of the chimney, the ambient temperature, ventilation or lack thereof in the room in which the exhausting appliance is located, objects near the chimney, the weather, and the diameter of the chimney.
A pipe chimney.
With residential chimneys, you primarily encounter three types of chimney: masonry, prefabricated, or pipe.
The simplest is the pipe chimney, which is a bare stovepipe which is routed directly from a stove out through the wall or ceiling of the house. The pipe is generally double- or triple-walled for insulation, and there are metal pass-throughs protecting where the pipe goes through walls or other combustible materials. Places where chimney pipe passes through a flat surface are often covered by protective storm collars which help to exclude water by directing runoff away from the joint. Pipe chimneys are usually used to provide ventilation for appliances, such as water heaters or boilers, or for freestanding wood stoves.
A prefabricated chimney.
A prefabricated chimney is really a prefabricated chimney system — it consists of a firebox/pipe chimney combo which are manufactured together and designed to be used together. The pipe chimney exits the house and is enclosed in an insulating chase (made of wood or masonry) which protects it from the elements (thus assisting in more efficient draw). The chase is topped by a chase cover, a flat metal “shoebox lid”, through which the flue pipe passes, and the pipe itself is topped with a single flue cap.
A masonry chimney.
Many older homes have a masonry chimney. These chimneys have an insulated masonry firebox at the bottom, in which the fire is burned or an insert is placed. Above the firebox is the smoke chamber, an area where the (usually large) firebox narrows into the actual interior of the chimney, called the flue. The tops of masonry chimneys are layered with mortar and are called crowns. The end of the flue generally protrudes through the crown, and the whole thing is topped with a single-flue or outside-mount chimney cap to keep out animals, debris, and water.
Since chimneys are essentially giant open tubes that draw air through them, they can affect temperature regulation of the home by allowing warm air to flow out of them in the wintertime. To prevent this, most chimneys have a moveable metal plate called a damper. The damper opens to allow gases out when a fire burns, or closes to keep warm air in the room when the fire is out. A throat-style damper “lives” around where the smoke chamber meets the flue. A top-sealing damper lives at the top of the chimney flue, and can also double as a chimney cap when it is in the closed position.
