The internal structure of a brick chimney

The mantel, the fireplace and wood logs gratings are components of a fireplace that almost all the world can identify. However, if you own a brick fireplace, you also need to know the names of the internal parts. Familiarize yourself with the structure from the inside out, which will give a better understanding of the components and mechanical parts that keep your fireplace working properly.


As the foundation of a house is important, it is also the basis of a fireplace. A correctly installed chimney will have a base – or shoe as it is called in some areas – that is independent of that of the house.

Home interior

The area directly below the fire is called the “inner hearth” and serves as the floor of the combustion chamber, which extends from the inside of the firebox into the room. The hearth area that is inside the room is known as the “outside hearth”. The floor of the interior fireplace is where the ashes are deposited after burning logs and should be routinely swept to remove them, once the ashes are no longer hot and the embers are extinguished.

Bump walls

The stop walls form the perimeter of the chimney and enclose the combustion chamber, starting from the base and rising to the bottom of the chimney at the point where its structure begins. Fireplaces and “zero space” units (prefabricated fireplaces installed just against the butt walls) are installed to fit just against the butt walls.

The combustion box

The flanks of the chimney that line the interior of the chimney form the “combustion box”, which is where you build fire with logs. Fireplaces and zero space units serve as a combustion box in homes where they are installed. In older homes with brick fireplaces, the section of the bumper walls that is visible from the inside of the room serves as the combustion box area. The combustion box – if it is an insert or the stop walls – must have the appropriate size and must be tilted so that the heat of the fire is emitted in the room.

Smoke platform

Air from the outside can create a downward current inside the chimney. The job of the smoke platform is to redirect the descending currents back into the pipe to come out of the chimney. The smoke platform has a curved upper surface for redirecting smoke and protrudes from the rear of the stop walls, which are located behind the gate and the combustion box.

Shock absorber

The damper is a plate or valve that regulates drafts when the duct is closed. It is operated manually and must be fully open before lighting a fire, which is normally effected by turning the fire control knob. The gate opens to ensure that the smoke rises through the chimney instead of in the room and directs the heat to flow into the room from the combustion chamber.

Chimney flue

The duct is the passage way of the chimney. The function of the duct is to create a stream so that the smoke rises to the chimney and out to the outside. The streams start and extend vertically from the combustion chamber to the top of the chimney. When chimneys are inspected, ducts are checked to ensure they are not leaking and are functioning properly.


Next to the fireplace, the fireplace is probably the most recognizable part of the structure, but many people are not quite sure where it starts. The vertical area that begins just after the smoke chamber is when it “officially” begins. The fireplace consists of the masonry walls surrounding the duct, the duct lining, the flashing and other mechanisms that are essential to make a fireplace work properly.