It is important to get the fireplace design and dimensions right. More about www wayback machine here. The 3 basic functions of any fireplace are defined by the following 3 characteristics: its fuel combustion, the difference between the air pressure within its firebox and the air pressure emanating from the chimney's top and the difference between the temperature of the air in the room where the fireplace is situated and the temperature of the air at the top of the chimney.
For an exhaust and combustion performance which is successful almost all of these design factors must be taken into consideration. The four basic design components of a fireplace are the firebox, the chimney chamber, the smoke chamber and the base of the fireplace.
The burning performance of a fireplace depends on the types of firebox, the chimney chamber and the smoke chamber. The base of the fireplace comprises of the heart support and foundation.
The Foundation: Fireplace foundations must be designed in such a way that it will be able to support the weight of the fireplace itself without resulting in any substantial or differential settlement. Most masonry fireplaces can be made of a structural slab or made with footings which lend support to the foundation walls.
Foundation footings are made of masonry or concrete materials and are usually twelve inches thick and they are designed to extend to at least six inches on all sides beyond the walls of the fireplace. Foundation walls are designed in such a way to ensure the fireplace is raised to the desired height and these are usually made of masonry of concrete materials at least eight inches thick.
These walls are designed without any cavities except for the ones catering to the external combustion air ducts and the pit for the ash which are created in the base area.
Structural slab foundations are meant to afford support to the fireplace assembly and are designed as such. It is sometimes necessary to make the slabs thicker below the fireplace in order to support its weight and that of the chimney.