Fireplaces: A construction primer

By The Old House Web

Figure 2: Masonry Fireplace Details

A fireplace adds to the attractiveness of the house interior, but one that does not "draw" properly is a detriment, not an asset.

By following several rules on the relation of the fireplace opening size to flue area, depth of the opening, and other measurements, satisfactory performance can be assured. Metal circulating fireplaces, which form the main outline of the opening and are enclosed with brick, are designed for proper functioning when flues are the correct size.

One rule which is often recommended is that the depth of the fireplace should be about two-thirds the height of the opening. Thus, a 30-inch-high fireplace would be 20 inches deep from the face to the rear of the opening.

Figure 1: Clearances for Wood Construction

The flue area should be at least one-tenth of the open area of the fireplace (width times height) when the chimney is 15 feet or more in height. When less than 15 feet, the flue area in square inches should be one-eighth of the opening of the fireplace. This height is measured from the throat to the top of the chimney. Thus, a fireplace with a 30-inch width and 24-inch height (720 sq. in.) would require an 8- by 12-inch flue, which has an inside area of about 80 square inches. A 12- by 12-inch flue liner has an area of about 125 square inches, and this would be large enough for a 36- by 30-inch opening when the chimney height is 15 feet or over.

The back width of the fireplace is usually 6 to 8 inches narrower than the front. This helps to guide the smoke and fumes toward the rear. A vertical back-wall of about a 14-inch height then tapers toward the upper section or "throat" of the fireplace (fig. 1). The area of the throat should be about 1 1/4 to 1 1/3 times the area of the flue to promote better draft. An adjustable damper is used at this area for easy control of the opening.

The smoke shelf (top of the throat) is necessary to prevent back drafts. The height of the smoke shelf should be 8 inches above the top of the fireplace opening (fig. 2). The smoke shelf is concave to retain any slight amount of rain that may enter.

Steel angle iron is used to support the brick or masonry over the fireplace opening. The bottom of the inner hearth, the sides, and the back are built of a heat-resistant material such as firebrick. The outer hearth should extend at least 16 inches out from the face of the fireplace and be supported by a reinforced concrete slab (fig. 2). This outer hearth is a precaution against flying sparks and is made of noncombustible materials such as glazed tile. Other fireplace details of clearance, framing of the wall, and cleanout opening and ash dump are also shown. Hangers and brackets for fireplace screens are often built into the face of the fireplace.

Figure 3: Dual Opening fireplace: A, Adjacent opening, B, through fireplace

Fireplaces with two or more openings (fig. 3) require much larger flues than the conventional fireplace. For example, a fireplace with two open adjacent faces (fig. 3A) would require a 12- by 16-inch flue for a 34- by 20- by 30-inch (width, depth, and height, respectively) opening. Local building regulations usually cover the proper sizes for these types of fireplaces.

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