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Title: Fireplaces and Chimneys

Author: Arthur H. Senner
T. A. H. Miller

Release date: February 28, 2015 [eBook #48378]
Most recently updated: October 24, 2024

Language: English

Other information and formats: www.gutenberg.org/ebooks/48378

Credits: Produced by Thomas Cosmas from materials made available
at The Internet Archive (https:/archive.org).

*** START OF THE PROJECT GUTENBERG EBOOK FIREPLACES
AND CHIMNEYS ***

THAT THE WORD "HEARTH" is synonymous with "home"
in many languages is not surprising since much of the enjoyment
of home and camp life centers about an open fire. In mild climates

a properly built fireplace will heat a single room, and when
equipped with a convection heater will also heat a second room
on the same floor or an upper floor. In colder climates it is a
useful adjunct to other heating systems if provided with a damper.
This bulletin is intended to give the householder and
prospective builder, especially the farmer who might superintend
the construction of his home, a working knowledge of the
principles to be observed in planning and building fireplaces and
chimneys. These principles, if observed, will make the structures
useful and satisfactory and insure their safety.
Safe fireplaces and chimneys that function properly can be
built by applying the principles given in this bulletin, but a good
chimney will not last indefinitely without proper care and repair.
Fireplaces and chimneys, being conspicuous architectural
features, should be pleasing in appearance and conform with the
general design of the building and its surroundings.
This bulletin supersedes Farmers' Bulletin 1649, Construction
of Chimneys and Fireplaces.
Washington, D. C. Issued December, 1941

FIREPLACES AND CHIMNEYS
By Arthur H. Senner, mechanical engineer, and
Thomas A. H. Miller, agricultural engineer. Division
of Farm Structure Research, Bureau of Agricultural
Chemistry and Engineering

CONTENTS

Page Page
Chimneys 2 Fireplaces—
Design 2 Continued.
Construction 7 Dimensions 35
Estimating brick 18 Cost estimate 43
Smoke test 18 Smoky fireplaces 45
Cleaning and repairing Outdoor fireplaces 46
19
flues Types 47
Fireplaces 22 Obtaining plans 48
Characteristics 22 Construction 48
Modified fireplaces 24 Operation 51
Selecting a fireplace 27 Barbecue pits 51
Construction 34 Dutch ovens 51

FIREPLACES AND CHIMNEYS should provide a safe place for an
open fire and a flue for draft to expel smoke from the fire passage to the
open air. They must be properly designed and constructed (fig. 1) if good
performance and protection against fire are to be obtained.

Figure 1.—A properly designed and well-built chimney that provides
ample draft and protection against fire.

CHIMNEYS

DESIGN
Solid masonry is the most satisfactory and safest material to use for
chimneys and fireplaces. If a chimney fire occurs, the safety of the building
may be dependent on the soundness of the flue walls (fig. 2). Cracked and
leaky flues not only are inefficient, destroying the draft as well as permitting
smoke and gases to pass into adjacent rooms, but are a dangerous fire
hazard. The chimney as known today was developed about 600 years ago.
Experience has shown that the satisfactory performance of a chimney flue is
determined by its size, direction, shape, height, tightness, and smoothness.

Draft
The draft of a chimney is the current of air created by the difference in
pressure resulting from variation in weight between the relatively hot gases
in the flue and the cooler outside air. The strength or intensity of the draft
depends, for the most part, on the height of the chimney, and the temperature
difference between the chimney gases and the outside atmosphere. The draft
is not so good in summer as in winter because the difference in temperature
between the outside air and the gases in the flue is less.
A very common error in chimney design is failure to distinguish between
the size of the flue required for free passage of the volume of smoke from a
given amount of fuel and that which, with proper height, will produce the
required draft. A chimney may be high enough (fig. 3), yet have an area too
small to expel the volume of smoke; or the size may be ample (fig. 4) but the
height not great enough to produce a strong draft. Either fault or a
combination of the two will result in unsatisfactory service.

Flue Sites
The dimensions of a flue for adequate draft depend principally on the
grate area and type of heating plant[1] and on the kind of fuel to be burned,
both of which should be determined before construction is begun. If a
chimney is found to be inadequate the only method of improving it, short of

reconstruction, is to increase its height. This is not always effective and is
often impracticable.
[1] Farmers' Bulletin 1698, Heating the Farm Home, contains information on
estimating the size of the heating plant needed for houses of different sizes and
for determining grate areas.

Table 1 gives the sizes of fire-clay flue linings ordinarily provided for
boilers, furnaces, stoves, or convection heaters burning soft coal. These sizes
have proved satisfactory for average flat-grate furnaces under normal
conditions. Manufacturers of heating equipment usually specify certain
requirements in chimney construction and will not guarantee the
performance of their heaters unless these requirements are met. Therefore
their recommendations should be followed when differing materially from
the dimensions given in this bulletin.

Height of Chimney
A chimney should extend at least 3 feet above flat roofs and 2 feet above
the ridge of peak roofs. Where chimneys cannot be built high enough above
the ridge to prevent trouble from eddies caused by wind being deflected
from the roof, a hood may be provided with the open ends parallel to the
ridge. Eddies which force air down the flues may be caused by building the
chimney too near trees (fig. 5, B) or a higher structure (fig. 6).

Figure 2.—Heavy masonry chimneys of this type are still being built
in rural areas. The thick walls, with unlined flues, are in good
condition after 75 years of continuous use.

Table 1.—Sizes of flue linings and heights of chimneys
recommended for flat-grate furnaces burning soft coal[A]

Nominal size of flue lining Height of
Round (inside Rectangular (outside chimney top
Grate
diameter) dimensions) at above grate at
area elevation
at elevation elevation
(Sq.
indicated indicated indicated
ft.)
Sea 2,000 4,000 6,000 Sea 2,000 4,000 6,000 Sea 2,000 4,000 6,000
Level feet feet feet Level ft ft ft Level feet feet feet
In. In. In. In. In. In. In. In. Ft. Ft. Ft. Ft.
8½ 8½ 8½
8½
1 8 8 8 10 by by by 2 26 32 36
by 13
8½ 8½ 8½
8½ 8½ 8½ 8½
2 10 10 10 10 24 29 35 41
by 13 by 13 by 13 by 13
8½ 8½ 13 by 13 by
3 10 10 12 12 26 33 41 49
by 13 by 13 13 13
13 by 13 by 13 by 13 by
4 12 12 12 12 30 37 45 49
13 13 13 13
13 by 13 by 13 by 18 by
5 12 12 15 15 32 37 43 52
13 13 18 18
18 by 18 by 20 by 20 by
6 15 18 18 18 30 37 47 56
18 18 20 20
20 by 20 by 20 by 20 by
7 18 18 18 18 32 41 49 64
20 20 20 20
20 by 20 by 20 by 20 by
8 18 18 18 18 35 42 56 10
20 20 20 20
[A] If anthracite is to be burned the area of the flue cross section may be
reduced about 25 percent.

The ratings given are based on comparatively smooth lined flues
with no offsets greater than 30° with the vertical.
The smallest sizes of fuels require excessive drafts and may
necessitate taller chimneys.
Flue heights and sizes are based upon approximately the several
altitudes indicated; it is sufficiently accurate to use the column giving
the altitude nearest to that of the particular problem.

When 2 or 3 appliances are connected to the same flue their total
grate area may be reduced 15 percent. The method of determining the
proper flue size for an altitude of 2,000 feet, when 1 appliance with a
grate area of 3 square feet and another with an area of 1.5 square feet
are attached to the same flue, is shown by the following example:
Add the 2 grate areas, 3 + 1.5 = 4.5 square feet. Reduce this total
area by 15 percent. Thus, 4.5 - 0.68 = 3.8 square feet is the required
area. Use the nearest whole number, 4. From the table it is seen that for
a grate area of 4 square feet at an elevation of 2,000 feet either a 12-
inch (inside diameter) round flue or a 13- by 13-inch (outside
dimensions) rectangular flue 37 feet high is required.

Figure 3.—This tall chimney
produced good draft for the
kitchen range, but the flue was
too small for a furnace. When
the house was remodeled, its
appearance was greatly
improved by building the
chimney inside.

Figure 4.—Short chimneys are frequently provided for low
bungalows, for architectural reasons. This flue is ample in size
but not high enough for use with a stove. The stone masonry has
been laid to harmonize with the rustic surroundings.

Frequently metal-pipe extensions are provided to increase the height of a
flue on account of the low cost and ease of installation, but these must be
securely anchored against wind and have the same area as the flue. Metal
extensions are likely to rust in a short time. They are available with a metal
cowl or top that turns with the wind to prevent air blowing down the flue.
Terra-cotta chimney pots or extensions are more durable and attractive.
A chimney located entirely inside a building has better draft because the
masonry retains heat longer when protected from cold outside air.

Figure 5.—Two pleasingly designed fireplace chimneys that fit into
their surroundings. A, This chimney stands in the clear and should
provide a good draft; B, a chimney under overhanging trees is
likely to backdraft. Contrast the appearance of these two
chimneys with that of figure 3.

Figure 6.—Several extensions were
necessary before this chimney
would draw properly on account of
the wind deflected from the nearby
wall.

Figure 7.—A, A good foundation extending below the soil affected by
frost. This chimney is well protected from ground moisture by the
concrete carried above the surface; B, an insecure foundation.
Supporting a chimney in this manner is a dangerous practice.

CONSTRUCTION

Supporting the Chimney
Stable foundations, preferably of concrete, should be provided, at least 6
inches wider all around than the chimney and 8 inches thick for one-story
and 12 inches thick for two-story houses. When there is no basement or
cellar (fig. 7, A), start the foundation of an exterior chimney well below the
frost line; otherwise, extend the base to the same level as the bottom of the
foundation of the building. Foundations for tall, heavy chimneys require
special consideration.
Where the wall of the house is of solid masonry 12 inches or more thick,
the chimney may be offset and carried on corbels or masonry brackets
instead of being carried down to the ground. The offset should not extend
more than 8 inches from the face of the wall, each course projecting not
more than 1 inch, and should not be less than 12 inches high. Often the
corbeling is started at the second- or third-floor level so that the chimney is
only one or two stories high.
Chimneys in frame buildings should be built from the ground up or
should rest on the foundation or basement walls if of solid masonry 12
inches or more thick.
A chimney resting on or carried by wooden floors, beams, or brackets or
hung from wooden rafters (fig. 7, B) is a fire hazard. Wood framing shrinks,
and beams supporting heavy loads deflect in time. Sagging beams injure the
walls and ceilings of the house and are apt to crack the chimney, rendering it
dangerous.

Flue Linings
Although, to save expense, chimneys are built without flue lining, those
with linings are more efficient. When the flue is not lined, the mortar and
bricks directly exposed to the action of fuel gases disintegrate. This
disintegration and that occurring from changes in temperature frequently
cause cracks in the masonry, thereby reducing the draft. An unlined chimney
is best if not plastered except at the sloped section (fig. 8, A). However, the
vertical and horizontal joints should be filled with mortar and struck smooth
and flush with the wall. Offsets or bends in flues (fig. 8) should not be

greater than 30°
with the vertical.
This slope can be
obtained by
offsetting or
corbeling each brick
course only 1 inch.
Flue lining must
withstand rapid
fluctuations in
temperature and be
resistant to the
action of ordinary
flue gases. The
shapes used as flue Figure 8.—For structural safety the amount of offset must be limited so
lining should be of that the center line, XY, of the upper flue will not fall beyond the
center of the wall of the lower flue. A, Offsetting of the left wall of
fire-clay, with shells an unlined flue is started two brick courses higher than on the right
not less than five- wall so that the area of the sloping section will not be reduced
eighths of an inch after plastering; B, a lined flue showing the method of cutting the
tile.
thick, and should be
vitrified. As a
safeguard against over-burning and brittleness, the lining should be tested by
submersion in water at room temperature for 24 hours, during which a
quantity of water weighing more than 3 percent of the dry weight of the
lining should not be absorbed. Place each length of flue lining in position,
setting it in cement mortar with the joint struck smooth on the inside, and
then lay the brick around it. If the lining is slipped down after several
courses of brick have been laid, the joints cannot be filled and leakage is
almost sure to result. Fill any spaces between the lining and the brickwork
completely with mortar, especially if the round type of flue is used.
The lower section of flue lining, unless resting on solid masonry at the
bottom of the flue, should be supported on at least three sides by brick
courses projecting to the inside surface of the lining. When laying brick and
lining, it is advisable to draw up a tight-fitting bag of straw as the work
progresses so as to catch material that might fall and block the flue.

Where offsets or bends are necessary in lined flues, tight joints can be
made by mitering or cutting equally the ends of abutting sections (fig. 8, B).
This can be done if a cement sack of damp sand is stuffed firmly into the
lining and a sharp chisel is tapped with a light hammer along the line where
the cut is desired. If the cutting is done after the lining is built into the
chimney, the lining may be broken and fall out of place. The hole for the
thimble can be cut the same way when a special thimble section is not used.
The linings commonly used are rectangular or round. Rectangular linings
are better adapted to brick construction than round linings, but the latter are
considered more efficient. The sizes commonly used are indicated in table 2.

Wall Thickness
Walls of chimneys not more than 30 feet high when lined should be 4
inches thick if of brick and reinforced concrete, 8 inches if of hollow
building units, and 12 inches if of stone. Linings may be omitted in
chimneys having walls of reinforced concrete at least 6 inches thick or of
unreinforced concrete or brick at least 8 inches thick, although lining is
desirable in the case of brick construction. Also the outside wall of a
chimney exposed to the weather is best made at least 8 inches thick.
In chimneys containing three or more flues, building codes generally
require that each group of two flues be separated from the other single flue
or group of two flues by brick divisions or withes not less than 3¾ inches
wide (fig. 9) . Where two flues are grouped without divisions, joints in the
linings of adjacent flues are safer if staggered at least 7 inches, and particular
care should be taken to have all joints filled with mortar. Individual flues are
advisable for fireplaces and heating furnaces or boilers.

Table 2.—Dimensions of commonly used standard commercial
flue lining

Rectangular linings[B] Round linings[C]
Outside Cross-sectional Inside Cross-sectional
Wall Wall
dimensions area diameter area
thickness thickness
(inches) Inside Outside (inches) Inside Outside

Square Square Square Square
inches feet Inches inches feet Inches
4½ by 8½ 23.6 0.26 ⅝ 6 28.3 0.29 ⅝
4½ by 13 38.2 .41 ⅝ 8 50.3 .49 ¾
7½ by 7½ 39.1 .39 ⅝ 10 78.5 .75 ⅞
8½ by 8½ 52.6 .50 ⅝ 12 113.0 1.07 1
8½ by 13 80.5 .78 ¾ 15 176.7 1.62 1⅛
8½ by 18 109.7 1.10 ⅞ 18 254.4 2.29 1¼
13 by 13 126.6 1.20 ⅞ 20 314.1 2.82 1⅜
13 by 18 182.8 1.70 ⅞ 22 380.1 3.48 1⅝
18 by 18 248.1 2.30 1⅛ 24 452.3 4.05 1⅝
20 by 20 297.6 2.60 1⅜ 27 572.5 5.20 2
[B] All rectangular flue lining is 2 feet long.
[C] Round flue lining, 6 to 24 inches in diameter, is 2 feet long; that 27 to 36
inches in diameter is 2½ or 3 feet long.

When two or more flues are used in unlined
chimneys, they must be separated by well-
bonded withes 8 niches thick. An attractive and
effective method of separating unlined flues in
colonial times is shown in figure 10.
Chimneys extending above the roof are
exposed to the wind and may sway enough
Figure 9.—Cross section of during a gale to open up the mortar joints at the
chimney showing the proper
arrangement for three flues. roof line. Openings in a flue at this point are
The division wall should be especially dangerous because sparks from the
well bonded with the side flue may come in contact with the woodwork of
walls by staggering the
joints of successive courses. the roof. It is therefore good practice to make the
Note the studs are kept 2 upper walls 8 inches thick (fig. 11) by starting to
inches away from the offset the bricks just below the intersection with
brickwork for reasons
explained on page 14. the roof.
The brickwork around all fireplaces and
flues should be laid with cement mortar, as it is more resistant than lime
mortar to the action of heat and flue gases. A good mortar to use in setting
flue linings and all chimney masonry, except firebrick, consists of 1 part

portland cement, 1 part hydrated lime, and 6 parts clean sand, measured by
volume. Slacked-lime putty may be used in place of hydrated lime; firebrick
is best laid in fire-clay.

Figure 10.—This Williamsburg chimney shows the pains taken to
make the chimney attractive. The three flues are arranged as a T
with well-bonded withes between them. Often four flues were
used in the form of a cross.

Openings Into the Chimney

No range, stove, fireplace, or ventilating register should be connected
with the flue being used for the heating apparatus because this is a frequent
cause of unsatisfactory operation. Fires may occur from sparks passing into
one flue opening and out through another where there are two connections to
the same flue. If an abandoned fireplace chimney is to be used for a range or
stove, close the fireplace flue tight about a foot below the smoke pipe hole.

Figure 11.—Greater resistance to the
weather is provided by building
the exposed upper section of a
chimney with 8-inch walls. Also
the mortar joint, in which the
counter-flashing is embedded, is
not so likely to fail as it is when
the wall is only 4 inches thick.

Gas-fired house heaters and built-in unit heaters, if not connected to a
masonry chimney, may be connected to flues of corrosion-resistant sheet
metal not lighter than 20-gage, properly insulated with asbestos or other
fireproofing material that will comply with the recommendations of the
Underwriter's Laboratories, Inc. Such flues should extend through the roof.

A soot pocket[2] is desirable for each flue. Deep pockets permit the
accumulation of soot, which may take fire; therefore start them from a point
preferably not more than 8 inches below the center line of the smoke pipe
intake and fill the lower part of the chimney with solid masonry instead of
extending the pocket to the base of the chimney as is often done. Clean-out

doors are necessary at the bottom of deep pockets and, if used, must fit
snugly and be kept tightly closed so that air cannot get in. Clean-outs should
serve only one flue, for if two or more flues are connected with the same
clean-out, air drawn from one to another affects the draft in all of them.
Sometimes a door is placed just below the smoke pipe, but one is not really
necessary since the pipe, if taken down each year for cleaning, allows
removal of soot from shallow pockets through the pipe hole.
[2] See soot pockets and clean-out doors in figures 12 and 35, pp. 13 and 38,
respectively.

Close pipe holes, when temporarily not in use, with tight-fitting metal
flue stops; but, if a pipe hole is to be abandoned, fill it with bricks laid in
good mortar. This stopping can be readily removed. The practice of closing a
pipe hole with papered tin is dangerous, for if there is another stove
connected with the flue, the metal may become hot enough to scorch the
unprotected wallpaper or even set it afire.
Proper care in setting and looking after pipe at its connection with the
chimney will greatly lessen the number of fires chargeable to defective
construction. Fit the pipe so that no opening will be left around it, and keep
it from projecting into the flue. The connection can be made airtight with a
closely fitting collar and boiler putty, good cement mortar, or stiff clay.
Smoke pipes should enter the chimney horizontally, and the hole through
the chimney wall to the flue should be lined with fire-clay, or metal thimbles
should be securely and tightly built in the masonry. Thimbles or flue rings
can be had of 6-, 7-, 8-, 10-, and 12-inch diameters and 6-, 9-, and 12-inch
lengths. If the walls are furred (fig. 12), the space between the thimbles and
the wood furring should be covered with metal lath and plaster.

Figure 12.—A, Connection to chimney
where furring is used. The brick
are built out around the thimble as

a protection against its cracking.
This is a fire hazard that is
frequently overlooked. B,
Connection when plaster is applied
directly to the masonry. Note that
the pipe extends too far into the
flue. It should be as shown in A.

When a smoke pipe is less than 18 inches from woodwork, the
woodwork requires protection against charring. A metal casing or asbestos
board 2 inches from the upper half of the pipe is sometimes employed to
protect woodwork directly above it. A pipe, even so protected, should never
be closer than 9 inches to any woodwork or other combustible material.
Commercial fireproof pipe coverings can be purchased.
If a pipe must be carried through a wood partition, protection for the
woodwork can be provided by cutting an opening in the partition and
inserting a galvanized-iron double-wall ventilating shield at least 12 inches
larger than the pipe (fig. 13) or by using at least 4 inches of brickwork or
other incombustible material. Smoke pipes should never pass through floors,
closets, or concealed spaces or enter a chimney in a garret.
Gases formed by burning the sulfur contained in coal are the main cause
of corrosion of metal smoke pipes. Little corrosion occurs during the heating
season, when the pipe is kept hot and dry.
The life of metal pipes can be prolonged if each summer when they are
not in use they are taken down, cleaned, wrapped in paper, and stored in a
dry place. This is especially true of pipe to heaters in damp cellars.

Figure 13.—A, Elevation of protection
around a stovepipe passing through a
frame partition; B, sectional view.

Insulation

No wood should be in contact with a chimney.
Leave a space of 2 inches between the outside face of
a chimney and all wooden beams or joists except
when 8 inches of masonry is used outside flue lining,
in which case the framing may be within one-half
inch of the chimney masonry. The space between the
floor framing and the chimney may be filled with
porous, nonmetallic, incombustible material, such as
loose cinders. Brickwork, mortar, and concrete are not
Figure 14.—Method of
suitable. Place the filling before the floor is laid, as it
insulating wood floor not only forms a fire stop but prevents accumulation
joists and baseboard of shavings or other combustible material.
at a chimney with 4-
inch walls. A single
Subflooring may be laid within one-half inch of the
header is used as it is masonry. Baseboards, when fastened to plaster that is
less than 4 feet long. directly in contact with the wall of a chimney, can be
protected by a layer of fireproof material, such as
asbestos, at least one-eighth of an inch thick between
the woodwork and the plaster (fig. 14).
Wooden studding, furring, or lathing should not be placed against a
chimney but set back, as indicated in figure 9; or the plaster may be applied
directly to the masonry or to metal lath laid over the masonry. The former is
the better method, as settlement will not crack the plaster. It is recommended
that a coat of cement plaster be applied directly upon the outside surfaces of
masonry chimneys that are to be incased by a wooden partition or other
combustible construction. Metal lath, lapped 6 inches on the masonry, at the
intersection of chimneys with partitions prevents corner cracks. (See plan in
fig. 34.)

Chimney and Roof Connection
Where the chimney passes through the roof, provide a 2-inch clearance
between the wood framing and masonry for fire protection and for expansion
due to temperature changes, settlement, or slight movement of the chimney
during heavy winds.

Figure 15.—Method of flashing. Sheet metal,
h, over the cricket, extends under the
shingles k, at least 4 inches and is counter-
flashed at l in joint. Base flashings b, c, d,
and e and cap flashings a, f, and g lap over
the base flashings and provide watertight
construction. A full bed of mortar should
be provided where cap flashing is inserted
in joints.

A chimney must be flashed and counter-flashed (fig. 15),[3] to make its
junction with the roof watertight. When the chimney is not located on the
ridge but on a sloping roof, a cricket, j, is built, as detailed in figure 16, high
enough to shed water around the chimney. Corrosion-resistant metal, such as
copper, galvanized metal, zinc, or lead, is best for the flashing and counter-
flashing. When tin is used, paint it well on both sides.
[3] See p. 26, Farmers' Bulletin 1751, Roof Coverings for Farm Buildings and
Their Repair, for method of installing flashing.

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.

Figure 16.—Cricket, j, as seen from
the back of the chimney shown in
figure 15. A section through the
cricket is also shown. Note how
counter-flashing is built into the
mortar joint at l.

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.

Figure 17.—A house in southern Maryland in which the space between the
chimney and the house wall shows clearly. The practice of building
the chimney in this way is common in the tidewater section of the
South.

Spark Arresters
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]
[4] See Standards for Construction and Installation of Spark Arresters for
Chimneys and Stacks, published by the National Fire Protection Association.

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.

Figure 18.—A, A common type of arched hood; B, flat stone hood; note the
withe separating the two flues.

ESTIMATING BRICK
The number of standard-size brick (8 by 3¾ by 2¼ inches) required to
build a straight chimney having only two or three flues can be estimated by
drawing the flue lining to scale and then drawing lines 4 inches to 8 inches
outside of the lining depending on the thickness of the brick walls. Lay out
4- by 8-inch rectangles in the space between the lining and the outside lines
to determine how many brick are needed per course.
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
Chimneys develop defects which if not promptly repaired cause trouble.
Most masonry requires replacement of worn or weathered material or
repointing of mortar joints, while flues become clogged and flashings fail.
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.
[5] Nichols, P., and Staples, C. W. REMOVAL OR SOOT FROM
FURNACES AND FLUES BY THE USE OF SALTS OR COMPOUNDS. U. S.
Bur. Mines Bul. 360, 76 pp., illus. 1932.

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.

Figure 19.—In 1744 Franklin promoted a metal fireplace of this type to be
set out into a room. These are known as Franklin stoves and sometimes
are equipped with andirons for burning wood or a grate for burning coal.
The metal blower, shown in front of the opening was used with grates
and set in place when starting the draft and then removed so that the
cheery heat of glowing coals could be enjoyed. At one time this type of
stove was highly thought of because it threw out more heat than the

built-in fireplace. A few manufacturers specialize in Franklin stoves
because of the present-day demand.

Tests conducted by this Bureau indicate that, as ordinarily constructed, a
fireplace is only about one-third as efficient as a good stove or circulator
heater. Nevertheless, they have a place as an auxiliary to the heating plant
and for their cheerfulness and charm. In milder climates, fireplaces may
suffice as the sole source of heat; also certain materials often wasted may be
utilized for fuel. The disadvantages of the ordinary fireplace are lessened by
"modified" fireplaces.

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).

Figure 20.—In this modified fireplace air enters the inlet, a, from outside and
is heated as it rises by natural circulation through the back chamber, c,
and the tubes, t, being discharged into the room from the register, b. Air
for supporting combustion is drawn into the fire at d and passes between
the tubes up the flue A damper is also provided to close the air inlet.

One advantage claimed for modified fireplace units is that the correctly
designed and proportioned firebox, manufactured with throat, damper,
smoke shelf, and chamber, provides a form for the masonry, thus reducing
the risk of failure and assuring a smokeless fireplace! However, there is no
excuse for using incorrect proportions; and the desirability of using a
foolproof form, as provided by the modified unit, merely to obtain good
proportions should be considered from the standpoint of cost. Even though
the unit is well designed, it will not operate properly if the chimney is
inadequate; therefore the rules for correct chimney construction must be
adhered to with the modified unit as well as with the ordinary fireplace.
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.

Figure 21.—In this fireplace the air is not drawn in
directly from outdoors but through the inlet, a,
from the room that is being heated. The air is
heated by contact with the metal sides and back
of the fireplace, rises by natural circulation,
and is discharged back into the room from the
outlet, b, or to another room on the same floor
or in the second story. The inlets and outlets
are connected to registers which may be
located at the front of the fireplace, as shown in
figure 22. The registers may be located on the
ends of the fireplace or on the wall of an
adjacent room.

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.

Figure 22.—Except for the registers and metallic sides and back, the
appearance of modified fireplaces is like that of ordinary ones. An
interesting effect is secured by the mirror—the reflection of the opposite
wall appears like a recess over the mantel.

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.

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).

Figure 23.—A well-designed commercial mantel that suits the room. Since it
is painted the same color as the walls, it does not focus attention, as the
handsomely carved formal mantel or mahogany shown in figure 37 is
intended to do.

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.

Figure 24.—Another good design is this revival of early New England
architecture, which is frequently used for remodeling public dining
rooms. The random-width pine planks were selected especially for
variety in the pattern of the knots. Note the use of otherwise wasted
space for bookshelves and closet.

Figure 25.—A, A fireplace at Mount Vernon, Washington's home, typical of
those used before cooking stoves were introduced. This type of
fireplace, if not too large, is often retained (B) when a kitchen is
remodeled into a living room. Note the Dutch oven at the right, formerly
used for baking.

Figure 26.—Fireplaces originally intended for wood were frequently bricked
up, and small cast-iron units of this type were built in, since the large
openings required for wood were wasteful when coal was used. This was
a very popular type of grate for hotel and private bedrooms about 1860
and can still be seen in old houses in coal regions. Note the plain and
neat mantel of wide plank.

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).

Figure 27.—This inexpensive fireplace 32 inches square shows how a plain
brick front can be used in a small room.

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.

Figure 28.—A, A shallow fireplace, with a copper hood, built as shown in B,
throws out considerable heat after the hood gets hot. The wall should be
of fire-resistant masonry.

Figure 29.—This shallow fireplace with a sloping back is a type that was
frequently built in bedrooms before the general use of stoves. Note the
neat and well-proportioned mantel.

Figure 30.—Screens are almost essential to protect the upholstery of nearby
furniture from sparks. This fireplace shows artistic use of small stones
and makes a pleasing contrast with the log walls.

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.

Figure 31.—This 36-inch-wide fireplace does not seem too large for the
small room, but its size would have been accentuated by the use of a
mantel.

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.

Figure 32.—An adobe fireplace of the Mexican-Indian type commonly built
in the Southwestern States, especially when the house walls are of
adobe. The logs are stood up, leaning against the back of the grate, in
order to secure a high-licking flame.

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]
Inside
Opening Outside diameter
Minimum Vertical Inclined
dimensions of
Depth, back back back
of standard standard
Width, Height, d (horizontal) wall, wall, rectangular round
w h c a b
flue lining flue
lining
Inches Inches Inches Inches Inches Inches Inches Inches
24 24 16-18 14 14 16 8½ by 8½ 10
28 24 16-18 14 14 16 8½ by 8½ 10
24 28 16-18 14 14 20 8½ by 8½ 10
30 28 16-18 16 14 20 8½ by 13 10
36 28 16-18 22 14 20 8½ by 13 12
42 28 16-18 28 14 20 8½ by 18 12
36 32 18-20 20 14 24 8½ by 18 12
42 32 18-20 26 14 24 13 by 11 12
48 32 18-20 32 11 24 13 by 13 15
42 36 18-20 26 11 28 13 by 13 15
48 36 18-20 32 14 28 13 by 18 15
54 36 18-20 38 14 28 13 by 18 15
60 36 18-20 44 14 28 13 by 18 15
42 40 20-22 24 17 29 13 by 13 15
48 40 20-22 30 17 29 13 by 18 15
54 40 20-22 36 17 29 13 by 18 15
60 40 20-22 42 17 29 18 by 18 18
66 40 20-22 48 17 29 18 by 13 18
72 40 22-28 51 17 29 18 by 18 18

Figure 33.—In regions where natural gas is plentiful and in cities, fireplaces
of this type, burning gas with a flickering flame, are frequently used as
an auxiliary to the main heating plant. Some types have imitation logs
of metal perforated for gas jets.

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.

Figure 34.—A typical fireplace, illustrating practical details of construction.
An alternate method of supporting the hearth is shown in the lower right-
hand corner. The various letters refer to specific features discussed in the
text.

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 (fig. 35).

Figure 35.—The ash-pit should be of tight masonry and should be provided
with a tightly fitting iron clean-out door and frame about 10 by 12 inches
in size. A clean-out for the furnace flue as shown is sometimes provided.

In buildings with wooden floors the hearth in front of the fireplace
should be supported by masonry trimmer arches (fig. 34) 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 (fig. 37).

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.

Figure 36.—Where a header is more than
4 feet in length, it should be doubled,
as shown. Headers supporting more
than four tail beams should have ends
supported in metal joist hangers. The
framing may be placed one-half inch
from the chimney because the
masonry is 8 inches thick.

Lintel
Lintels of ½- by 3-inch flat iron bars. 3½- by 3¼- by ¼-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 (fig. 38) 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
The sides of the fireplace should be vertical up to the throat, or damper
opening (ff fig. 34). The throat should be 6 to 8 inches or more above the
bottom of the lintel and have an area not less than that of the flue and a
length equal to the width of the fireplace opening. Starting 5 inches above
the throat, ee, the sides should be drawn in at tt to equal the flue area.
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.

Figure 37.—Grates of this type are commonly used in fireplaces for burning
coal or coke. This one has a metal back and ends and is only set in to
permit proper circulation of air around it.

The smoke chamber is the space extending from the top of the throat, ee,
up to the bottom of the flue proper, tt, and between the side walls. The walls
should be drawn inward 30° to the vertical after the top of the throat, ee, is
passed and smoothly plastered with cement mortar not less than one-half
inch thick.

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.

Figure 38.—This well-designed small stone fireplace was built in
accordance with the principles given in this bulletin. It is a good heater
and does not smoke. The jambs are wide enough to resist the thrust of
the arch.

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.

Figure 39.—Diagram showing front view and cross section of an entire
chimney such as is commonly built to serve a furnace, fireplace, and
kitchen stove. Two sets of dimensions are given, those in rectangles refer
to the approximate sizes of the voids or openings; the others refer to the
outside dimensions of the brickwork. These are used in estimating the
number of bricks in a chimney. The letters A-F indicate sections used in
estimating the quantities of brick required (See p. 44.)

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 (table 2). 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 (128 square 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 (p. 7) apply to fireplace flues.
Table 4 is convenient in selecting the proper size of flue or for
determining the size of fireplace opening for an existing flue. The area of the
fireplace opening in square inches is obtained by multiplying the width, w,
by the height, h, (fig. 34), both measured in inches.

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. [6]
[6] Inches and fractions of an inch are converted to feet and decimals by
multiplying by 0.0833; thus 2 X / inches × O.0833 equals 0.208 feet.

Table 4.—Sizes of fireplace flue linings[D]
Outside
Inside
Area of dimensions
diameter of
fireplace of standard
standard round
opening rectangular
flue lining
flue lining

Square
Inches Inches
inches
600 8½ by 8½ 10
800 8½ by 13 10
1,000 8½ by 18 12
1,200 8½ by 18 12
1,400 13 by 13 12
1,600 13 by 13 15
1,800 13 by 18 15
2,000 13 by 18 15
2,200 13 by 18 15
2,400 18 by 18 18
2,600 18 by 18 18
2,800 18 by 18 18
3,000 18 by 18 18
[D] Based on a flue area equal to one-twelfth the fireplace opening. Sec table
2 for areas of flue lining.

Number of Bricks
(1) Estimate the total volume of masonry by multiplying together the
length, width, and height of the various sections (fig. 39).
Length Width Height Volume
Section
Feet Feet Feet Cubic feet
AB 6.0 by 2.75 by 12.66 = 209.0
BC 4.25 by 2.5 by 1.66 = 17.6
CD 3.5 by 2.0 by 2.0 = 14.0
DE 3.5 by 1.75 by 10.16 = 62.2
EF 4.33 by 2.5 by 6.0 = 65.0
Total volume including voids 367.8
(2) Estimate the total volume of voids by multiplying together their
length, width, and height.
Length Width Height Volume
Item = = = =
Feet Feet Feet Cubic feet

Ash-pit 2.33 by 1.5 by 7.0 = 24 46
Fireplace 3.0 by 1.5 by 3.5 = 15.75
Smoke chamber 2.0 by 1.16 by 2.0 = 4.64
8½- by 13-inch flue[E] 0.78 square feet by 28 5 = 22.23
13- by 13-inch flue[E] 1.20 square feet by 18.75 = 22.50
8½- by 8½-inch flue[E] .50 square feet by 18.75 = 9.37
Total volume of voids 98.95
[E] See table 2 for outside areas of flues in square feet.

(3) Subtract volume of voids from volume of masonry.
Cubic feet
Total volume, including voids 368
Total volume of voids 99
Total volume of masonry 269
(4) Multiply net volume of masonry by the number of brick per cubic
foot.
269 by 22.5 = 6,053 brick, or 6.1 thousand bricks.
Mortar
To estimate the mortar needed, multiply the mortar material given below
for 1,000 brick by 6.1 to determine how much will be needed to build the
chimney, using 1:1:6 mixture recommended on page 10.
Bags of hydrated lime 2.6 by 6.1 = 16 bags.
Sacks of portland cement 3.5 by 6.1 = 22 sacks.
Cubic feet of sand 18.0 by 6.1 = 110 cubic feet = 4 cubic yards.
Foundation
Concrete needed for foundation can be estimated as follows: Concrete
for foundation should be 1:2½:5 and for the top 1:2½. The foundation is 7 by
3.75 by 1, or 26.25 cubic feet, or 1 cubic yard, and will require 5 sacks of
cement, 0.46 cubic yard of sand, and 92 cubic yard of gravel. The cap is 4.5
by 2.66 by 0.5 = 5.9 cubic feet The area of the three flues above must be
deducted: 5.9 minus 2.48 = 3.42 cubic feet, or one-ninth of a cubic yard. As

1 cubic yard was assumed for the foundation, extra cement and sand are not
needed. Other material needed:
1 8-inch thimble, 9 inches long.
1 6-inch thimble, 9 inches long.
28 feet of 8½- by 13-inch flue lining.
20 feet of 13- by 13-inch flue lining.
20 feet of 8½- by 8½-inch flue lining.
Damper, 36- by 10-inch throat opening.
2 clean-out doors and 1 ash dump.
Mantel as selected.
If firebrick is to be used or the exposed breast is to be of face or special
brick (or ceramic tile) the number should be counted or estimated and
deducted from the number of common brick as estimated above.

Labor
The labor required to build a chimney depends on the thickness of the
walls, the height, and the amount of cutting to build in specialties, provide
offsets, etc. In general, a mason will take 16 hours with 8 hours of laborer's
help to lay 1,000 brick. On this basis, 16 by 6.1 = 97.6 hours of mason's time
and 48.8 hours of laborer's time will be required.

Cost
The approximate cost of the chimney can be determined by using actual
local cost of materials and wages as follows:[F]
6,100 brick at $15.00 per thousand $91.50
27 sacks of cement[G] at $0.70 per sack 18.90
16 bags of lime at $0.50 per bag 8.00
5 cubic yards of sand[G] at $2.25 per cubic yard 11.25
1 cubic yard of gravel at $2.00 per cubic yard 2.00
98 hours, mason's time, at $1.00 per hour 98.00
[H] 14.70
49 hours, laborer's time, at $0.30 per hour
28 linear feet of 8½- by 13-inch flue at $1.00 per foot 28.00
20 linear feet of 13- by 13-inch flue at $1.15 per foot 23.00

20 linear feet of 8½- by 8½-inch flue at $0.40 per foot 8.00
1 8-inch thimble .60
1 6-inch thimble .40
2 clean-out doors
Damper, lintel mantel, ash dump
65.00 }
Total net cost [I] 369.35
[F] The prices used in this example are merely illustrative.
[G] Includes material for footing and cap.
[H] Includes labor for footing and cap.
[I] Where the chimney is built by contract, 10 to 15 percent should be added
for profit and overhead.

SMOKY FIREPLACES
When a fireplace smokes, it should be examined to make certain that the
essential requirements of construction as outlined in this bulletin have been
fulfilled. If the chimney is not stopped up with fallen brick and the mortar
joints are not loose, note whether nearby trees or tall structures cause eddies
down the flue. To determine whether the fireplace opening is in correct
proportion to the flue area, hold a piece of sheet metal across the top of the
fireplace opening and then gradually lower it, making the opening smaller
until smoke does not come into the room. Mark at the lower edge of the
metal on the sides of the fireplace. The opening may then be reduced by
building in a metal shield or hood across the top so that its lower edge is at
the marks made during the test; or the trouble can generally be remedied by
increasing the height of the flue.

OUTDOOR FIREPLACES
Outdoor fireplaces range from simple makeshifts to elaborately equipped
structures harmonizing with the architecture of the house.
No one type will meet all conditions, but all types should be practical to
use and yet not be fire hazards or eyesores.

Figure 40.—A, A fireplace built for 30 cents, cash. One hundred and twenty
bricks and six concrete blocks were picked up a few at a time along the
road. One sack of cement was purchased, one-half of which was used for
another job Sand was available on the site. B f Detailed drawings show
dimensions of this fireplace. As the fireplace is ordinarily built, the
material would cost about $5 and the labor from $5 to $10, depending on
local conditions.

TYPES
The tendency is to build too large an outdoor fireplace. Where only a
little cooking is to be done occasionally in a small yard or at a picnic,

several concrete blocks or stones set on the ground about 12 to 16 inches
apart will serve. The shelf of an old refrigerator may be used for a grille. If
permanence is desired, the walls should be laid in cement mortar and the
fireplace should have a suitable foundation and a permanent grille. An end
wall is recommended to prevent embers from being scattered by drafts
blowing between the side walls. Smoke annoyance while cooking is
lessened by making the fireplace long enough to permit a short chimney
(fig. 40).

Figure 41.—An outdoor fireplace built back of an inside fireplace and
opening onto a paved terrace provides comfort in early fall.

A circle of stones laid loosely on the surface, larger stones set partly into
the ground, or carefully laid masonry walls on a stable foundation may be
used for campfires and small barbecue parties. A cast-iron pot with a lid can
be buried in the ashes for baking. Pipe supports for pots and pans built into
the masonry are a convenience; they can be homemade or purchased. Spits
for roasting can be improvised or bought.
Fireplaces opening onto an enclosed porch or paved terrace, are often
built as an integral part of the house chimney (fig. 41). The corner of
boundary walls permits effective treatment. Such fireplaces should meet the

regulations of local fire authorities and be built with the same care and be
subject to the same rules as inside fireplaces.

OBTAINING PLANS
Plans for outdoor fireplaces are available from various publishing
houses; several magazines feature illustrations that can be adapted to the
material at hand. If a structure is to be built with local labor and material,
simple designs are advisable. The size of stones, joints, and proportions
have a direct influence upon appearance, and good personal taste frequently
results in more pleasing structures than blind adherence to conventional
designs. The various combinations of ovens, cranes, grilles, storage
compartments, benches, lights, sinks, etc., to be used as built-in features
affect the design. Before planning a structure with these features, catalogs
of dealers in outdoor fireplace equipment should be consulted for sizes of
the available accessories so that ample space and proper details can be
provided in the masonry for building them in. Skilled labor should be
employed for elaborate designs (fig. 42) when much equipment is built in or
when the fireplace, as in figure 41, is an integral part of a permanent
building.

Figure 42.—This fireplace, set at a focal point in the garden, enhances the
landscape. It was built by a skilled mason.

CONSTRUCTION
Ordinarily the fire is built on the hearth, no grate being used. Fire
regulations in hazardous localities may require firing doors, dampers, spark
screens, and a solid-plate cooking surface; otherwise these features are not
essential. Two and a half square feet of cooking surface is desirable, while
access to both sides and the end permit several people to cook at the same
time. The side walls should have fairly level tops for pots and pans. Side
walls are made 2 to 6 inches higher than the cooking level to permit
anchoring the grille; if too high, they interfere with cooking. Commercial
grilles are available, but satisfactory ones can be made of ½-inch to ¾-inch
pipe or ⅝-inch reinforcing rods. The pipes should be 6 to 10 inches longer
than the width of the firebox; they should be spaced not more than 1¼
inches apart and have their tops exactly level to prevent pots and pans from
wobbling. Two or three pipes can be used for a lintel over the opening into
the flue if regular iron lintels are not available. Where a solid top is desired,
it should be of boiler plate at least ¼-inch thick. Such plates must be
stiffened to prevent buckling by alternate heating and cooling; for ordinary
purposes they are merely set on top of the grid though they may be hinged
at the rear so they can be tipped back against the chimney.
The best draft is secured when the fireplace faces the direction of
prevailing breezes and is protected from strong winds which might scatter
sparks. If the fireplace is built too near shrubbery or under trees, the heat
and smoke may damage or burn the foliage. A slight rise or a gentle slope
that affords good drainage should be selected. Paving the ground around the
fireplace, with flagstones or covering it with a layer of gravel or sand will
prevent the area from becoming a mudhole or an unsightly bare spot; also,
danger of starting brush fires by sparks falling from the firebox is lessened.
Fireboxes 12 to 16 inches wide, 16 to 24 inches long, and 6 to 8 inches
deep with the hearth at 9 to 16 inches above the ground are sufficient for
most purposes. Large fireboxes are wasteful of fuel; while, if the grille is
too high above the hearth, much of the best cooking heat from glowing
coals is lost. Most grilles are set 15 to 24 inches above the ground, though
30 inches may be desirable to avoid the necessity of stooping when

cooking. The hearth should slope 1 to 2 inches toward the front so that rain
water will drain away.
The area of the chimney flue should be at least one-eighth the vertical
cross-sectional area of the firebox.
Fire-clay linings for the firebox and flue are not absolutely necessary
except when required by fire regulations or where hot fires are maintained
for long periods. They, or common brick linings, are advisable for the more
permanent and expensive structures or where it is necessary to use porous
stone, such as sandstone and most stratified rocks, which absorb water and
flake or chip upon exposure to fire.
Most rocks or stone that can be worked up without special tools or skill,
brick, and concrete are adaptable for the average fireplace (fig. 43). The
size of the stones determines the thickness of the walls; no wall should be
less than 8 inches thick.
Where suitable stone is difficult to get in sufficient quantity, the exposed
surface may consist of a shell the thickness of the stone and the inner
portion of the wall be made of concrete or large stone bedded in concrete.
When flue lining is necessary, it will serve as a form for the flue: otherwise
a metal stovepipe makes a practical form, or the flue can be formed of brick
laid on edge. The stones can be laid and the concrete deposited with the
least trouble by building the veneer and flue only 6 to 8 inches high at one
time. All masonry should be laid in mortar, as described on page 10.
Concrete made in the proportions 1:2½:4 will serve most purposes where
wall sections are about 8 inches thick. For heavy foundations and thick
walls not subject to direct fire, 1:3:6 concrete is strong enough.
A concrete slab 4 to 8 inches thick with the bottom 4 to 6 inches below
the surface provides a sufficient foundation for medium-sized structures
where frost is negligible and the soil is well drained and firm. If the soil is
not well drained a 6- to 8-inch layer of stone, cinders, or sand should be
provided under the slab and the surface of the immediate vicinity graded or
otherwise protected from water. It is advisable to use ¼-inch or ½-inch
reinforcing rods, 6 inches apart in both directions, one-third the distance
from the top in slabs for all but the smallest fireplaces or where frost is not
severe.

Figure 43.—A, Ground plan of a stone fireplace that can be built in different
sizes to suit the landscape; B, vertical-section sketch; C, the completed
fireplace. The stones for this fireplace were picked up at "the swimming
hole." About 8 bushels were used over a backing of concrete; 16 bushels
would have been needed if the concrete had not been used. Gravel and
sand were dug from the excavation. The chimney is battered 4 inches in
the 24-inch height.

Heavy and expensive structures, especially those having tall chimneys,
should have foundations below the surface affected by frost or erosion and
strong enough to prevent settlement or cracks. Such a foundation can be
made of concrete, with a liberal use of large stones for economy, extending
under the whole structure, or be continuous walls with a footing similar to
those used for houses. The advice and help of builders may save money in
the construction of foundations of large expensive fireplaces, especially

where climatic conditions are severe and the bearing power of soil is not
known.

OPERATION
Cooking should be done over glowing coals, as flames and smoke
smudge the utensils or even the food. When the wood is nearly charred,
most of the smoke has been driven off and the chimney is hot enough to
draw the smoke up the flue. All fires should be banked before they are left,
to avoid setting fires. Banking can be safely and effectively done by raking
the coals and unconsumed fuel into a pile on the hearth and covering the
pile with a few inches of earth. Water thrown on a hot fire may result in
scalds from the steam and may crack the fireplace.

BARBECUE PITS
Barbecue ovens are rather expensive unless for community use.
Outdoor fireplaces, without or with spits for roasting, are frequently called
barbecues in some sections of the country. For occasional barbecuing
parties, a hole in the ground will serve. Dig a hole several feet deep and
several feet larger each way than the size of the carcass to be roasted; then
place stones in the bottom to retain the heat. A trench 30 inches deep, 36
inches wide, and about 10 feet long will accommodate about 400 pounds of
beef. A fire should be built sufficiently ahead of time, about 3 hours, to heat
the stones and bottom and accumulate ashes for proper banking. Have
someone with previous experience operate the pit, because improper
wrapping of the meat and handling of the coals results in poor cooking.

DUTCH OVENS
Dutch ovens (fig. 25) are often built in connection with both indoor and
outdoor fireplaces to copy early kitchen fireplaces or for actual baking.
When used as an ornament, the oven is fitted with a cast-iron door, and
the space thus formed may be used for wood storage. An open firebox or

compartment below may be similarly used. Spaces used for wood storage
should be separated from the fireplace by a brick or stone partition at least 8
inches thick, all joints being completely filled with mortar.
If the oven is intended for baking, it is advisable to line it with firebrick,
and the masonry should be at least 8 inches thick. A greater heat-storing
capacity is secured by using thick walls. An ash drop of standard cast-iron
unit type is provided for modern ovens and may lead either to the side of
the fireplace or to an ash-pit in the chimney base. The throat or dome
should be carefully formed with brick molded or ground to an arch and
preferably should be fitted with a damper. If the top of the oven is flat,
several lintel irons will be needed to support the brick. A separate flue with
a damper is recommended. For ovens of ordinary size an 8½- by 8½-inch
flue is ample. The oven is preheated by fire or hot coals. Before food is
placed in the oven, the coals and ashes are removed through the ash drop.
Figure 44 shows a Dutch oven made of tapered adobe bricks and
plastered outside with adobe mud. A hole in the top permits the escape of
smoke, while the orño is being heated by the fire built inside on the floor.
This hole is closed with an adobe block after the coals are raked out. When
loaves of bread have been placed on the floor with a wooden paddle, the
door hole is stopped with adobe brick.

Figure 44.—This Dutch oven of adobe bricks is
a type very common in the States along the
Mexican border. It is called an orño and is
usually built a short distance from the
kitchen door.

Boy Scouts and campers frequently improvise Dutch ovens by packing
damp sandy clay, 8 to 12 inches thick, around a wooden barrel, a tin wash
boiler, or slabs of rock to form a vault. After the earth has been gradually
dried and baked with a slow fire, the oven is ready for use. The hole in the
top and the door can be closed as in the orño.

U. S. GOVERNMENT PRINTING OFFICE: 1947

For sale by the Superintendent of Documents, U. S. Government Printing
Office
Washington 25, D. C. - Price 15 cents

Transcriber Note
Paragraphs split by illustrations were rejoined. The footnotes for
the tables were changed from arabic numerals to alphabetic
characters to distinguish them from the text footnotes. The cover
image was derived from materials made available at The Internet
Archive and is placed in the Public Domain.

*** END OF THE PROJECT GUTENBERG EBOOK FIREPLACES AND
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