It is important to get the fireplace design and dimensions right. 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 hearth 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 deired 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.
The hearth support can also be supported by using corbelled brickwork. You can also use cantilevered reinforced brick masonry o reven a structural slab of concrete .
The firebox is made up of a smoke shelf, the combustion chamber, throat and of course the fireplace opening. With regards to the hearth there are the 2 primary parts, the extended hearth and also the inner hearth. The floor of the combustion chamber is created by the inner hearth which is located within the firebox area.
Projecting outwards into the room located behind the fireplace face is the extended hearth which is also noncombustible.
The opening of the fireplace is a major part of the design of the fireplace and most of the other components that make up the fireplace are based of the dimension and configuration of its opening.
The shape of the fireplace opening is also important, because the higher the opening, the more it will increase the glowing heat and also increase the demand for more air in the room and will definitely require a taller chimney.
The bricks used for support above the fireplace opening must have adequate support such as brick arches, reinforced brick masonry lintels, stone, pre-cast concrete and loose angle lintels. The dimensions of the firebox should be designed in such a way to allow the fire to fill the combustion chamber when it is lighting, as this will provide better heating effectiveness.
A firebox opening should be designed at the proper size because if the opening is too small, although it might function normally, it will not be able to provide enough heat to keep the room warm, in the same manner if it is too big, it will require a greater flue area and of course will use up huge amounts of the interior air.
The shape and depth of the combustion chamber plays a significant part with respect to the combustion air requirements and the amount of heat reflected and radiated into the room and will also influence the draft in the room.
