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Masonry Fireplaces

by admin on July 27th, 2010

Masonry fireplaces utilize a masonry building unit (brick, cement, stone, etc.) as their main material for construction. Within masonry fireplace construction, a variety of types exist, each with differences from one another:

  • Radiant – Radiant fireplaces heat by the emission of infra-red radiation. The fire heats the fireplace, which in turns heats the surrounding area through infra-red radiation. A typical design picture is available.
  • Air Circulation – Air-circulation fireplaces work by heating the air that crosses the hot surface of the fire. This warm air then rises and, in doing so, attracts more air up behind it, creating a circulation effect. This method is achieved through the use of panels or ducts that are built into the fireplace. These ducts attract air from the area, and circulate it around a metal firebox, warming it. This warm air is then released into the area. Air-circulating fireplaces warm quickly and cool down even quicker.
  • Metal Insert – Metal-insert fireplaces are relatively new and are installed in many newly built homes. Metal fireboxes are placed in the framing of the house and a flue is placed through the roof or side of the house. These fireplaces usually are made to work with gas logs only.
  • Multi-Flue – Multiple-flue fireplaces have two or more flues. A flue, as elaborated upon in the “Methods and Materials” section below, is the duct or vent that rises from the fireplace hearth to allow the noxious gases from the fire to be released. Multi-flue fireplaces can also be radiant, circulating or metal insert. Sometimes more than one flue is utilized if the fireplace’s size is great, but more often multiple flues are strictly built for design purposes.

An understanding of the numerous components and terms applicable to masonry fireplaces is necessary to understand how a fireplace is built and designed.

  • The hearth of a fireplace is the open recess of the fireplace.
  • The firebox of a fireplace is the area within the hearth that the materials are gathered and set ablaze on the hearth floor.
  • A mantel is the hood or decorative framework that surrounds a fireplace. It allows great leeway for design customization.
  • A flue is the upward opening or duct that allows gasses from the fireplace to exit safely. It is the space within a chimney which carries the gasses upwards. Though they serve an important function in transporting noxious gasses away from the home, they can also allow much heat to escape when a fire is lit.
  • A damper is the valve that regulates the flow of air and gasses within a chimney. It is a plate-like piece that can be opened or closed by the fireplace user. When the fireplace is not in use, it is useful as it stops air from escaping and stops rain or snow from entering the flue. When the damper is open, it allows for gasses to escape through the flue. Dampers should always be open when the fireplace is in use.
  • The grate is the metal frame object within the fireplace and holds the materials that are set ablaze.
  • Fireplace caps are the units which are placed atop the chimney, and directly over the flue. They usually consist of a screened siding to prevent animals from entering the chimney, and an overhead solid covering to prevent moisture or air infiltration. As they are usually visible along the rooftops, they are often times elaborately designed or decorated. They are most commonly made of steel, stainless steel, aluminum and copper.

Fireplaces are not load-bearing, structural elements. Instead, they are self supporting and spread on concrete foundations.

Fireplaces are not an energy-efficient way of heating. In fact, only 15-20 percent of the warmth a fireplace generates will actually circulate throughout the home. The rest escapes through the flue within the chimney and disappears into the outside air. Fireplaces are also prone to creative negative energy, as warm or cool air from the home can escape if the damper is not closed. One easy step to take to avoid the loss of air from your home is to have sealed-glass doors guarding the fireplace instead of a wire or mesh cover. If a masonry fireplace is to be installed in harsh weather conditions, special precautions should be taken.

Frequent and detailed chimney inspections are highly recommended for the safety of the homeowners and the structural integrity of the building. As parts of the fireplace unit are exposed to outside forces, this is of special concern. Any cracks or splits within the chimney should be reported to local fireplace and chimney inspection crews.

Components within the fireplace are designed with function and safety in mind. Dampers are designed to open fully and close tightly to ensure that gasses can escape and warm air can remain when the fireplace is not in use. It accomplishes this with a metal plate-like unit that has numerous bars within the middle section. These bars then pivot to the open or closed position.  A picture is available.

Fireplace caps are placed at the top of the chimney, and are designed to keep out animals and to prevent moisture from entering the fireplace and causing damage. Caps are made with a variety of materials, with the most popular options being stainless steel and copper. They consist of a screened siding and a solid covering above. Caps do not come in one standard size or shape, as they must be custom fit to be applied to the chimney they are covering.

In the case of extremely warm or cold weather conditions, some precautions should be taken. Masonry units used should be refractory, or appropriately fire-rated to withstand the heat from the fire. Charts and further information is available from the Masonry Advisory Council. Also, the mortar used should be appropriate to the project and should be able to withstand the high heat it must endure. These guidelines are designed to increase efficiency of the fireplace but, more importantly, to ensure the fireplace is able to operate safely.

Firebox walls, the surfaces that will come under the most scrutiny and intensity from the fire, should have the following composition: A 4″exterior brick layer, followed by a middle section consisting of 4″ concrete masonry units, and finishing with 2″ firebrick with ¼″ refractory mortar joints as an interior layer.

Dampers are required by building safety codes to be at least 8 inches above the top of the fireplace opening. They should be placed accordingly. Flues should be sized in cooperation with the size of the fireplace opening. The Masonry Advisory Council recommends the following guidelines:

The size of the hearth should also be carefully considered. It is recommended that the hearth should extend, from front of the fireplace opening to back, 16 inches if the opening is 6 square feet or less. If the opening is greater than 6 square feet, a 20-inch hearth extension should be constructed. The hearths width, from each side of the opening, should be 8 inches if the fireplace is 6 square feet or less, and 12 inches if larger than 6 square feet.