Read Ebook: Fireplaces and Chimneys by Miller T A H Thomas Arrington Huntington Senner Arthur H Arthur Henry

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Manufacturers claim labor and materials saved tend to offset the purchase price of the unit; also that the saving in fuel justifies any net increase in first cost. A minimum life of 20 years is claimed for the type and thickness of metal commonly used today in these units.

Field tests made by this Bureau have proved that, when properly installed, the better designs of modified-fireplace units circulate heat into the cold corners of rooms and will deliver heated air through ducts to adjoining or upper rooms. For example, heat could be diverted to a bathroom from a living-room fireplace.

The quantity and temperature of the heated air discharged from the grilles in figures 20 and 21 were measured to determine the merits of the convection features. These measurements showed that very appreciable amounts of convected heat are produced by the modified unit when properly installed and operated. Discharge-air temperatures in excess of 200? F. were attained from some of the units tested. The heated air delivered from the discharge grilles of some of the medium-sized units represented a heating effect equivalent to that from nearly 40 square feet of cast-iron radiation of the ordinary hot-water heating system, or sufficient to heat a 15- by 18-foot room built with average tightness to 70? F. when the outside temperature is 40? F. Additional convected heat can be produced with some models by the use of forced-circulation fans.

However, the nature of operation, with the unavoidably large quantity of heated air passing up the stack, makes the inherent over-all efficiency of any fireplace relatively low. Therefore, claims for an increased efficiency of modified fireplaces should be understood merely as constituting an improvement over the ordinary fireplace and not over stoves or central heating plants.

When a fireplace is being selected the kind of fuel to be burned should be considered; also, the design should harmonize with the room in proportion and detail .

In colonial days, when cordwood was plentiful, fireplaces 7 feet wide and 5 feet high were common, especially when used in kitchens for cooking . They required large amounts of fuel and too frequently were smoky.

Where cordwood is cut in half, a 30-inch width is desirable for a fireplace; but, where coal is burned, the opening can be narrower . Thirty inches is a practical height for the convenient tending of a fire where the width is less than 6 feet; openings about 30 inches wide are generally made with square corners. The higher the opening, the greater the chance of a smoky fireplace.

In general, the wider the opening the greater should be the depth. A shallow opening throws out relatively more heat than a deep one of the same width but accommodates smaller pieces of wood; thus it becomes a question of preference between a greater depth which permits the use of large logs that burn longer and a shallower depth which takes smaller-sized wood but throws out more heat.

In small fireplaces a depth of 12 inches will permit good draft if the throat is constructed as explained above, but a minimum depth of 16 to 18 inches is advised to lessen the danger of brands falling out on the floor.

As a rule, fireplaces on the second floor are smaller than those on the first floor and it is well to follow this practice because the flue height is less for second floor fireplaces .

Unless a fireplace 6 feet wide is fully 28 inches deep, the logs will have to be split, and some advantage of the wide opening will be lost.

Screens of suitable design should be placed in front of all fireplaces .

A fireplace 30 to 36 inches wide is generally suitable for a room having 300 square feet of floor . The width should be increased for larger rooms, but all other dimensions should be taken from table 3 for the width selected.

The corner of a room often is the favorite location for a fireplace . Fireplaces of the type shown in figure 28 are also built in corners.

Units providing for burning gas are often built in to resemble fireplaces .

Pleasing designs result from exercising good taste in use of materials and mantels that suit the room. The photographs in this bulletin have been selected to illustrate various architectural effects that can be developed and should help in the choice of a type suitable for houses of different designs. The essentials for safety and utility, however, should not be sacrificed for style.

CONSTRUCTION

The ordinary fireplace is constructed generally as shown in figure 34. It is essential that the flue have the proper area, that the throat be correctly constructed and have suitable damper, that the chimney be high enough for a good draft, that the shape of the fireplace be such as to direct a maximum amount of radiated heat into the room, and that a properly constructed smoke chamber be provided.

DIMENSIONS

Table 3 gives recommended dimensions for fireplaces of various widths and heights.

If a damper is installed, the width of the opening j, figure 34, will depend on the width of the damper frame, the size of which is fixed by the width and depth of the fireplace and the slope of the back wall. The width of the throat proper is determined by the opening of the hinged damper cover. The full damper opening should never be less than the flue area. Responsible manufacturers of fireplace equipment give valuable assistance in the selection of a suitable damper for a given fireplace. A well-designed and well-installed damper should be regarded as essential in cold climates.

When no damper is used, the throat opening j should be 4 inches for fireplaces not exceeding 4 feet in height.

Table Key

Footings

Footings for chimneys with fireplaces should be provided as described on page 7; for chimneys without fireplaces, the footings should rest on good firm soil.

Hearth

The hearth should be about flush with the floor, for sweepings may then be brushed into the fireplace. When there is a basement, an ash dump located in the hearth near the back of the fireplace is convenient. The dump consists of a metal frame about 5 by 8 inches in size, with a plate, generally pivoted, through which ashes can be dropped into a pit below .

In buildings with wooden floors the hearth in front of the fireplace should be supported by masonry trimmer arches or other fire-resistant construction. Hearths should project at least 16 inches from the chimney breast and should be of brick, stone, terra cotta, or reinforced concrete not less than 4 inches thick. The length of the hearth should be not less than the width of the fireplace opening plus 16 inches. Wooden centering under trimmer arches may be removed after the mortar has set, though it is more frequently left in place. Figure 36 shows a recommended method of floor framing around a fireplace.

Wall Thickness

The walls of fireplaces should never be less than 8 inches thick, and if of stone they should be at least 12 inches thick. When built of stone or hard-burned brick, the back and sides are often not lined with firebrick, but it is better to use firebrick laid in fire-clay. When firebricks are laid fiat with the long sides exposed there is less danger of their falling out. They are generally placed on edge, however, forming a 2-inch protection, in which case metal ties should be built into the main brickwork to hold the 2-inch firebrick veneer in place. Thick metal backs and sides are sometimes used as lining. When a grate for burning coal or coke is built in, firebrick at least 2 inches thick should be added to the fireplace back unless the grate has a solid iron back and is only set in with an air space behind it .

Jambs

The jambs should be wide enough to give stability and a pleasing appearance; they are frequently faced with ornamental brick or tile. For an opening 3 feet wide or less, a 12- or 16-inch width is generally sufficient, depending on whether a wood mantel is used or the jambs are of exposed masonry. The edges of a wood mantel should be kept at least 8 inches from the fireplace opening. For wider openings and large rooms, similar proportions should be kept.

Lintel

Lintels of 1/2 - by 3-inch flat iron bars. 3 1/2 - by 3 1/4 - by 1/4 -inch angle irons, or damper frames are used to support the masonry over the opening of ordinary fireplaces. Heavier lintel irons are required for wider openings.

Where a masonry arch is used over the opening, the jambs should be heavy enough to resist the thrust of the arch. Arches over openings less than 4 feet wide seldom sag, but sagging is not uncommon in wider fireplaces, especially where massive masonry is used.

Throat

Proper throat construction is so necessary to a successful fireplace that the work should be inspected several times a day during construction to make certain that the side walls are carried up perpendicularly until the throat is passed and that the full length of opening is provided.

Smoke Shelf and Chamber

The smoke shelf is made by setting the brickwork back at the top of the throat to the line of the flue wall for the full length of the throat. Its depth may vary from 6 to 12 inches or more, depending on the depth, d, of the fireplace.

Damper

A properly designed damper, as shown in figure 34, affords a means of regulating the draft and prevents excessive loss of heat from the room when the fire is out. A damper consists of a cast-iron frame with a lid hinged so that the width of the throat opening may be varied from a closed to a wide-open position. Various patterns are on the market, some designed to support the masonry over the opening, others requiring lintel irons.

A roaring pine fire may require a full-throat opening, but slow-burning hardwood logs may need only 1 or 2 inches of opening. Regulating the opening according to the kind of fire prevents waste of heat up the chimney. Closing the damper in summer keeps flies, mosquitoes, and other insects from entering the house down the chimney.

In houses heated by furnaces or other modern systems, lack of a damper in the fireplace flue may interfere with uniform heating, particularly in very cold windy weather, whether or not there is a fire on the hearth. When air heated by the furnace is carried up the chimney there is a waste of the furnace fuel, but a damper partially open serves a slow fire of hardwood without smoking the room or wasting heated air from the main heating system.

Flue

The area of lined flues should be a twelfth or more of the fireplace opening, provided the chimney is at least 22 feet in height, measured from the hearth. If the flue is shorter than 22 feet or if it is unlined, its area should be made a tenth or more of the fireplace opening. The fireplace shown in figure 34 has an opening of 7.5 square feet, or approximately 1,080 square inches, and needs a flue area of approximately 90 square inches; a rectangular flue, 8% by 18 inches, outside dimensions, or a round flue with a 12-inch inside diameter might be used, as these are the nearest commercial sizes of lining . It is seldom possible to obtain lining having exactly the required area, but the inside area should never be less than that prescribed above. A 13- by 13-inch flue was selected for convenience when combining with the other flues. If the flue is built of brick and is unlined, its area should be approximately one-tenth of the fireplace opening, or 108 square inches. It would probably be made 8 by 16 inches because brickwork can be laid to better advantage when the dimensions of the flue are multiples of 4 inches. The principles of construction given under Chimneys apply to fireplace flues.

COST ESTIMATE

A convenient method for estimating the number of bricks in a chimney is to calculate the volume of the various sections which differ in outside dimensions and then subtract the voids or cavities resulting from ash-pits, fireplace, and flues. This will be the total cubic feet of brickwork which, when multiplied by 22.5, is converted to number of bricks. For convenience, inches as indicated in figure 39 have been converted to decimals of a foot.

Inches and fractions of an inch are converted to feet and decimals by multiplying by 0.0833; thus 2 X / inches x O.0833 equals 0.208 feet.

Based on a flue area equal to one-twelfth the fireplace opening. Sec table 2 for areas of flue lining.

Number of Bricks

Estimate the total volume of masonry by multiplying together the length, width, and height of the various sections .

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