Fireplaces and Chimneys

A feature, said to have originated in colonial Williamsburg as a precaution against fire hazard, is to build the upper section of outside chimneys 18 inches to 2 feet away from the gable ends of the house (fig. 17). This is not only a safety factor but a practical one because the chimney can be more easily flashed, small windows can be used in the walls of upper story rooms behind the chimney, and framing the roof is simplified.

Capping the Chimney

Various methods of terminating chimneys are shown in figures 11 and 18. Whatever one is used should be architecturally acceptable, effective in preventing disintegration, and so made as to keep water out of the flue.

It is advisable to project the flue lining 4 inches above the cap or top course of brick and surround it with at least 2 inches of cement mortar finished with a straight or concave slope to direct air currents upward at the top of the flue; the sloped mortar also serves to drain water from the top of the chimney. (See fig. 11.) Hoods are commonly used to keep rain out of a chimney (fig. 18, A and B). The area of the hood openings should be at least equal to the area of the flue and each flue should have a separate hood. Concrete and brick caps are usually made 4 inches thick, and it is advisable to project them an inch or two to form a drip ledge.

Many of the chimneys built today are unsightly and frequently detract from an otherwise well-designed house. Within the last 100 years the size and attractiveness of chimneys ordinarily built has declined. The large old chimneys of colonial days were proportioned to suit the house and surroundings and at the same time provide for two or more large fireplaces. With reduction in the size of fireplaces and the substitution of several stoves and eventually one central heating plant, the chimney has developed into a merely utilitarian shaft.

Spark arresters are desirable and, where chimneys are near combustible roofs, lumber, forests, etc., they are sometimes required, depending on the kind of fuel, waste materials, or refuse that may be burned and the amount of deposits that may accumulate in the flues. While arresters cannot be depended on to eliminate entirely the discharge of sparks under all conditions; yet, when properly built and installed, they materially reduce spark hazard.[4]

In general all parts, whether of wire, expanded metal, or perforated sheets, give longer service if they are of rust-resistant material. Arresters for domestic purposes should have vertical sides extending upward not less than 9 inches so as to provide a gross area of surface at least twice the net flue area. They should be kept outside of the flue area and be securely anchored to the chimney top.

Openings in the screen not larger than five-eights of an inch nor smaller than five-sixteenths of an inch are advisable. Commercially made screens can be purchased which generally last several years. Arresters must be kept adjusted in position and renewed when the openings are worn larger than the normal screen openings.

ESTIMATING BRICK

For example, 15½ brick are needed for each course of the chimney in figure 9. Assuming the height is 30 feet and one-half-inch mortar joints are used, also that there are 4½ courses per foot, there would be 135 courses. Therefore, 135 multiplied by 15½ equals 2,092 brick; about 100 more will be needed to make the lower portion solid, or 2,200 brick in all.

A more general method of estimating that is applicable to more complex structures is given on page 43. Methods of determining the quantity of mortar materials, labor, and cost are also given and can be applied to this example.

SMOKE TEST

Every flue should be subjected to the following smoke test before the heater is connected with it and preferably before the chimney has been furred and plastered or otherwise enclosed. Build a paper, straw, wood, or tar-paper fire at the base of the flue. When the smoke is rising in a dense column, tightly block the outlet at the top of the chimney with a wet blanket. Smoke that escapes through the masonry indicates the location of leaks. Frequently this test reveals bad leaks into adjoining flues or directly through the walls or between the linings and the wall. Remedy defects before the chimney is accepted for use. Such defects are usually difficult to correct; hence it is wise to watch the construction closely as it progresses.

CLEANING AND REPAIRING FLUES

It is advisable to test a chimney every few years for tightness by the smoke test just described; to examine the inside of the flues by lowering a lantern or flashlight on a strong cord down from the top of the chimney or by holding a hand mirror at the proper angle at a stovepipe hole; to inspect the masonry for loose units, which are most likely to occur at the top (fig. 3) where the action of the flue gases, especially when soft coal is burned, disintegrates the mortar; to test mortar joints from the outside by prodding with a knife or similar tool to determine if the mortar is loose clear through the joint so as to leave a hole; and to notice if the chimney is damp because of leaky flashings, absorption of moisture from the ground, condensation, or excessive rain entering the flues.

Cleaning

Bricks that fall from the top and lodge at offsets or contracted sections can sometimes be reached and dislodged by a long pole or sections of pipe screwed together. They can be caught on a shingle or piece of sheet metal shoved into a stovepipe hole or removed through a clean-out door. A weighted cement sack filled with straw and attached to the end of a rope may be pulled up and down the flue to remove soot and loose material if the offset is not too great.

Trouble with creosote and soot can be reduced when one understands how they are formed. Smoke and soot are caused by imperfect combustion, usually due to one or all of the following conditions: (1) Lack of sufficient air to the fire; (2) improper mixture of air with furnace gases; (3) low furnace temperature; (4) too small combustion space so that the gases reach the comparatively cool furnace surface before they are completely burned and, as a result, soot or tarry matter condenses and then passes up the chimney in the form of smoke. Soft coal causes more soot trouble than hard coal.

If soot accumulates fast or trouble is experienced with unusual smoke when firing, it is probable that the heating equipment is not being operated properly. The manufacturer or installer usually is able to suggest proper adjustments.

Investigations by the United States Bureau of Mines[5] have shown that various materials on being burned or volatilized form a vapor or smoke which settles upon soot; causing it to ignite at a lower temperature and burn more easily. For soot to burn, the gases in contact with it must have a temperature high enough to ignite it and sufficient air to support the combustion. The effectiveness of burning varies with the composition of the remover, but it also depends upon conditions being favorable. It will usually reduce somewhat the soot in a furnace and smoke pipe but not in a chimney. It has no effect on the ash mixed with the soot. This ash not only does not burn, but prevents complete burning of the soot mixed with it.

Soot removers cause soot to burn and are fire hazards. The correct and most thorough method of cleaning a chimney is to do so manually or to employ modern exhaust or vacuum methods used by furnace repairmen. However, it is inconvenient to remove soot and ash accumulations thoroughly more than once a year; hence a remover may help to keep the passages of stoves and heaters clear between annual cleanings, if deposits of soot accumulate quickly and reduce the draft.

Likelihood of success in cleaning is greater when the deposits of soot are thick, provided they do not cut down the draft too much. If burning is employed, there is less risk when it is done frequently enough to prevent large accumulations, which cause intense fires. Also, freeing the heater and pipe of soot permits better fuel burning and higher temperatures in the chimney flue, thus reducing the amount of soot likely to be deposited on the flue walls.

Common salt (rock or ice-cream salt) is not the most effective remover, yet it is the most widely used because of its cheapness, ease of handling, and general availability. Use two or three teacupfuls per application. Metallic zinc in the form of dust or small granules is often used; however, a mixture of salt and 10 percent zinc dust is more effective than either salt or zinc alone.

One of the most effective mixtures of materials readily available is 1 part dry red lead and 5 parts common salt, measured by weight. Shake these together in a can with a tight-fitting lid. As lead is poisonous, wash the hands after using. One or two teacupfuls are used per application.

Old dry-cell batteries contain suitable ingredients and when they are thrown in a hot furnace the soot usually burns. Quicker action can be had if they are chopped up.

Before a remover is used, the fire must be put in good condition with a substantial body of hot fuel on top. Close the ash-pit door and the slots in the firing door and scatter the remover on the hot coals. Close the firing doors and at once reduce the draft by partially closing the pipe dampers. The draft should not be closed so tight as to cause fumes to escape into the cellar. Let the remover "stew" for 10 to 20 minutes or until fumes stop rising from the coals; then make the fire burn fiercely by opening the ash-pit door and the damper. Shaking ashes out will help. The slots in the firing door can be opened or the door itself set ajar. If soot in the furnace will not ignite, throw a little wood or paper on the fire.

Instead of making a special job of cleaning at intervals, one or two cups of salt may be thrown on the fire once a day with the expectation that the furnace will produce a high enough temperature to ignite some of the soot. This is most likely to succeed in cold weather when the furnace temperatures are high.

Cause of Creosote

Creosote is the result of condensation in the chimney, and trouble from this source is best avoided by preventing creosote formation. It is more likely to form when wood is used for fuel than when coal is burned and is more likely to form in cold than in mild climates. Green wood may contain as high as 40 percent water, and dry wood 15 to 20 percent. When wood is slowly burned, it gives off acetic and pyroligneous acid, which in combination with water or moisture form creosote. When the draft is strong and an active fire is maintained, much of the creosote is carried off into the atmosphere. The trouble is aggravated when the fire does not burn briskly and when an outside flue is subjected to chilling blasts. The walls of the chimney, being comparatively cool, cause condensation of the vapors contained in the smoke. Thus the creosote condenses and runs down the flue, finding its way out of any joints that are not perfectly tight. The formation of creosote is unusual in chimneys that are surrounded by warm rooms. The outer walls of a chimney in an outside wall should be at least two bricks thick and the chimney should have a good flue lining.

Creosote is difficult to remove and when it ignites makes a very hot fire that is likely to crack the masonry and char adjacent timbers. The only safe method of removal is to chip it from the masonry with a blade or straightened-out hoe attached to a pipe or handle. A heavy chain drawn up and down the flue walls is sometimes effective. However, when creosote is removed, care is necessary not to knock out mortar joints or to break the flue lining.

Large quantities of salt thrown on the fire in the grate or fireplace will extinguish a chimney fire. A fire in a fireplace flue can be checked in its intensity and frequently extinguished by first quenching the fire on the hearth and then holding a wet rug or blanket over the opening so as to shut off the air. When this is done, the soot and creosote are likely to slide from the flue walls and drop into the fireplace. Before extinguishing a fire in a flue, cover openings into the rooms, so that the soot will not spread over furnishings.

Repairing Chimneys

When a chimney is damp, examine the flashing at the junction with the roof, especially if wet spots appear on the ceilings of rooms. Methods of repairing flashing are given in Farmers' Bulletin 1751, Roof Coverings for Farm Buildings and Their Repair. If the flashing is sound, possibly water runs down the inside of the flue and through defective mortar joints. Where these cannot be reached readily, the chimney may have to be torn down and rebuilt. Sometimes a hood (fig. 18, A and B) is built on top of the chimney to keep out water or to prevent wind blowing down it. To prevent dampness being drawn up from the ground, the mortar can be raked from a joint at least 12 inches above the ground and a layer of slate, asbestos shingles, or rust-resistant sheet metal and new mortar worked into the joint. This work should be done by a mason. If bricks are porous or eroded, raking out the mortar one-half of an inch deep and applying three-fourths of an inch of cement plaster to the surfaces is effective. Eroded joints in the rest of the masonry should be raked and repointed. Where natural gas is burned, dampness due to condensation is not unusual and a drain may be needed. Where such conditions exist, advice should be sought from the manufacturers of the equipment as to the proper remedy.

A chimney that becomes too hot to permit holding the hand against it should be carefully inspected by a reliable mason and adequately protected as suggested in the preceding pages.

If, after a chimney is cleaned, an examination discloses holes, unfilled joints, or other unsound conditions out of reach for repair, it is advisable to tear the masonry down and rebuild properly. Inside bricks that are impregnated with creosote and soot should not be used in the new work because they will stain plaster whenever dampness occurs. It is almost impossible to remove creosote and soot stains on plaster and wallpaper. Sometimes painting the plaster with aluminum-flake paint or waterproof varnish hides the stains.

A hatchway cut through a roof is convenient when high chimneys are repaired or cleaned, especially when access to the roof is difficult. The hatchway should be located so that it will not be necessary to crawl over the roof to reach the chimney and so that a ladder placed on the attic floor will not be too steep for safe ascent. A watertight cover with hooks to prevent its blowing off is essential. Such a hatchway is best provided when the building is erected but can be readily built at any time.

FIREPLACES

A fireplace is ordinarily considered appropriate to a living room, dining room, and bedroom; however, basement, porch, and outdoor fireplaces are gaining in favor with the householder. Also public dining places, offices, etc., frequently have fireplaces for the comfort and for the air of informality they provide.

All fireplaces should be built in accordance with the few simple essentials of correct design given herein if satisfactory performance is to be realized. They should be of a size best suited to the room in which they are used from the standpoint of appearance and operation. If too small, they may function properly but do not throw out sufficient heat. If they are too large, a fire that would fill the combustion chamber would be entirely too hot for the room and would waste fuel.

The location of the chimney determines the location of the fireplace and too often is governed by structural considerations only. A fireplace suggests a fireside group and a reasonable degree of seclusion, and therefore, especially in the living room, it should not be near doors to passageways of the house.

CHARACTERISTICS

The principal warming effect of a fireplace is produced by the radiant heat from the fire and from the hot back, sides, and hearth. In the ordinary fireplace practically no heating effect is produced by convection, that is, by air current. Air passes through the fire and up the chimney, carrying with it the heat absorbed from the fire; at the same time outside air of a lower temperature is drawn into the room. The effect of the cold air thus brought into the room is particularly noticeable farthest from the fire. Heat radiation, like light, travels in straight lines, and unless one is within range of such radiation, little heat is felt. Tests made by the Bureau of Agricultural Chemistry and Engineering showed that about five times the amount of air required for even liberal ventilation may be drawn into a living room by the operation of a fireplace. Such excessive ventilation may cause chilling drafts. Persons located at advantageous points in the room will be comfortable under such conditions, but those out of the radiation zone will not.

MODIFIED FIREPLACES

The Franklin stove (fig. 19) is a type of modified fireplace.

The modified fireplaces of today are of several types, as shown in figures 20 and 21.

Both the last two types of modified fireplaces are manufactured as units of heavy metal, designed to be set into place and concealed by the usual brickwork, or other construction, so that no practical change in mantel design is required by their use. The modifications are built-in standard parts of the fireplace—only the grilles show (fig. 22).

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.

SELECTING A FIREPLACE

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 (figs. 23 and 24).

In colonial days, when cordwood was plentiful, fireplaces 7 feet wide and 5 feet high were common, especially when used in kitchens for cooking (fig. 25). They required large amounts of fuel and too frequently were smoky.

Where cordwood (4 feet long) is cut in half, a 30-inch width is desirable for a fireplace; but, where coal is burned, the opening can be narrower (fig. 26). 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 (fig. 27) 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 (fig. 28, A and B) 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 (fig. 29).

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 (fig. 30).

A fireplace 30 to 36 inches wide is generally suitable for a room having 300 square feet of floor (fig. 31). 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 (fig. 32). 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 (fig. 33).

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 (1) that the flue have the proper area, (2) that the throat be correctly constructed and have suitable damper, (3) that the chimney be high enough for a good draft, (4) that the shape of the fireplace be such as to direct a maximum amount of radiated heat into the room, and (5) 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 3.—Recommended dimensions for finished fireplaces

[Letters at heads of columns refer to figure 34]