Size the flue to provide enough draft after sizing the fireplace to match the space.
A fireplace needs acceptable dimensions and a proper relationship between the fireplace area, flue size, and chimney height in order to appear appealing and operate effectively.
Despite the wide variety of designs, there are several widespread patterns that have arisen as a result of successful fireplaces. A fireplace that burns poorly or doesn’t follow building codes may be the result of deviating too far from accepted procedures.
The Size of the Opening
The size of a masonry fireplace opening is mostly a design choice that is affected by the size of the space and the fireplace’s intended visual importance. The suitable fire size for the room and its intended functions is another consideration.
A large fireplace could overpower a tiny space and provide an uncomfortable amount of heat. A too-small one might not be visually striking and generate insufficient heat to be enjoyable. According to one guide, the fireplace opening should occupy between 130 and 165 of the room’s space. For small spaces, use the 1:30 ratio; for large rooms, use the 1:65 ratio (Ref. 1). The height-to-width ratio of a fireplace aperture should be between 1:1 and 1:2.
A tall, narrow entrance is more likely to let smoke into the room, and it is also more aesthetically pleasing when the opening is bigger. Standard fireplaces must be at least 20 inches deep according to BOCA Basic Building Code and the Uniform Building Code (UBC).
It is safer to presume that only the firebrick burning area is included in this depth because the standard regulations are ambiguous about whether the facing material is also included.
There are some fireplaces with multiple open faces. Two neighboring faces are present on the corner or L-shaped units. The look-through type with two opposing faces and three-face typefaces are other designs. To avoid draft issues, designs for multi-opening fireplaces should be carefully researched. In addition to flue size, think about the kind and size of damper you’ll need as well as the placement of walls and doors. Avoid places where the fireplace’s smoke and sparks could enter the room due to cross drafts.
Flue Sizes Stated by Building Codes
You can choose the suitable flue size once the fireplace opening size has been established. The inadequate draft will result from a too-small flue. A fire may burn more vigorously than necessary if the flue is excessively large. Using an enormous flue wastes materials and space even if this tendency can be controlled by regulating the damper. The number of drafts increases when the chimney height is increased for a given fireplace opening and flue size.
In accordance with the area of the fireplace opening, building codes often stipulate a minimum cross-sectional flue size. With a minimum size stated, the most typical example is 1/10 of the fireplace opening area. The UBC only requires 1/10 the area (minimum 64 square inches) for square or rectangular flue liners and only 1/12 the area (minimum 50 square inches) for round flue liners.
If the chimney is less than 15 feet tall, the Federal Housing Administration (FHA) mandates flue liners 1/8 in the area of the fireplace opening; for chimneys 15 feet and above, the requirement is 1/10.
Effective Flue Area
Local variations might be seen in the types of flue liners that are offered.
Because they make it easier to install reinforcing steel at the chimney corners, round or oval liners are frequently used in seismic regions.
Because exhaust gases have a tendency to spiral upward, the draft is more effective in a round or oval flue than in other designs with the same cross-sectional area. Because soot and creosote gather more easily in the corners of rectangular flues, round or oval flues may also require less maintenance.
In fact, circular or oval liners are advised to be used whenever possible, according to the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE), the chart’s source. When employing square or rectangular liners, you must determine the effective flue area (EFA) by subtracting the corners from the interior measurements of the liner. For instance, the effective flue area of a 10-inch-square flue is the same as the area of a 10-inch-diameter circle.
An oval flue with equal length and width and a rectangular flue has the same EFA. Determine the area of the rectangle between them as well as the half circles that are tangent to each end. The EFA for a rectangular flue measuring 10 by 14 inches would be equal to the area of the circle (0.7854 x 102 = 78.54 square inches) plus the area of a rectangle measuring 10 by 4 inches (10 x 4 = 40 square inches), for a total area of 118.54 square inches.
Applying the Chart
The chart offers a quick method of determining the ideal flue size for a specific fireplace opening. Calculate the width and height of the openings in the chart’s left and lower halves, respectively.
Consider the breadth to be the biggest region through which air will be carried for fireplaces with several openings:
• Use the diagonal space between corners for an L-shaped fireplace with two neighboring openings.
• Add the widths of each hole for a look-through fireplace with two opposite faces open.
• To calculate the width of a fireplace with two long openings and one short opening, sum their widths.
• To calculate the width of a fireplace with one long and two short sides, add their respective widths. Until they intersect, trace the lines from each dimension. From their intersection, draw a line horizontally to the right until it intersects the diagonal line that most closely represents the chimney’s height. The flue area required to ensure proper draft is shown as the vertical bar inside which the horizontal (opening area) line meets the diagonal (chimney height) line.
Masonry Chimney Foundation
It is necessary to have a stable concrete footing that can withstand the weight of the fireplace and/or chimney. To find out the minimum specifications for all footings and reinforcements, see the local code. Use concrete for fireplaces that are the right type and design.
Note: the chimney should not support any other parts of adjacent structures unless specifically designed to do so.