How Indoor Wood Burning Fireplaces Work

Masonry Hearth, Firebox and Damper

• Although many homeowners use the word "hearth" to refer to the entire visible portion of a fireplace, hearth technically refers to the floor surface just outside the fireplace's enclosed box. The enclosed box, which holds fuel and fire, is called a "firebox." Above the firebox, and between the firebox and chimney, is a tilting plate, called a damper. Fire requires air draft for combustion. Moving the damper adjusts the draft into the firebox and regulates combustion.

Masonry Fireplace — Above the Damper

• In a masonry fireplace, smoke rises beyond the damper and enters a holding area called the "smoke chamber." The smoke chamber's sloping sides narrow toward the chamber's top. A flat masonry shelf, called the "smoke shelf," sits at the rear of the smoke chamber. Above the chamber and shelf, a portion of the chimney called the "flue" shoots straight through the roof. The draft pulled into the chimney not only fuels the fire's flames, but also assists in pulling smoke from the chimney. When cold, outside air contacts the smoke shelf, convection causes warm air and smoke to rise through the flue and to the building's exterior.

Prefabricated Fireplaces

• Prefabricated fireplace units are relatively simple compared to masonry fireplaces. Prefabricated units usually consist of metal components; a metal firebox, damper and chimney. Designed for efficiency and safety, prefabricated fireplaces usually do not require smoke chambers and smoke shelves. As with masonry fireplaces, the prefabricated unit's firebox houses the fire, and the chimney circulates air and vents smoke.

Circulating Fireplaces

• Circulation attachments or equipment are a common addition to prefabricated fireplace units. Basic circulating fireplaces feature vents or openings that provide hot air additional escape routes into a room. Rather than flowing up and out a chimney, excess hot air blows into an interior space. Alternatively, many circulating fireplaces have fans that mechanically force excess hot air into a room.