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Title: Farmers' Bulletin 1230 - Chimneys & Fireplaces - They Contribute to the Health Comfort and Happiness of the - Farm Family - How to Build Them
Author: Daniels, A. M.
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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They Contribute to the Health Comfort and Happiness of the Farm Family



OF THE mistakes commonly made in home building none is more frequent than
faulty design and construction of chimneys and fireplaces. Though the use
of the fireplace is one of the oldest methods of house heating there are
few who understand the principles of its action, and even experienced
masons frequently fall into errors in building which seriously detract
from the efficiency of the installation. No defect in the construction
of the house detracts more from the comfort of the home and none is a
greater menace to life and property than a poor chimney and fireplace.
Bad chimney design is also the cause of much avoidable expense in heating
the house.

This bulletin is designed to give the householder and prospective
builder, and especially the farmer or other rural resident who builds or
superintends the building of his own home, a working knowledge of the
principles to be observed in planning and building these important parts
of the house, which, if they are observed, will go a long way to promote
the comfort of the home and insure the safety of the property.

Contribution from the Bureau of Public Roads THOS. H. MacDONALD, Chief

Washington, D. C. Issued, December, 1921; reprint, April 1922


A. M. Daniels, _Assistant Mechanical Engineer, Division of Agricultural
Engineering, Bureau of Public Roads_.




      Function of chimneys                           3
      The chimney draft                              4
      Shapes and sizes of flues                      5
      Height of chimney                              7
      Flue linings                                   7
      Location and wall thickness                    8
      Openings into the chimney                     10
      Supporting the chimney                        10
      Capping the chimney                           12
      Chimney and roof connections                  12
      Chimney connections                           12
      Chimney insulation                            14
      Smoke test for leakage                        14
      Cleaning and repairing the flue               15


      Essentials of fireplace construction          15
      Area of the flue                              16
      The throat                                    17
      Smoke shelf and chamber                       18
      Shape of fireplace                            19
      Throat damper                                 19
      Placing the throat damper                     20
      Size of fireplace opening                     20
      Depth of fireplace opening                    20
      The hearth                                    21
      The jambs                                     21
      Fireplace back and sides                      21
      Supporting irons                              21
      Improving fireplace heating                   22


[Illustration: T]

HE prime function of a chimney is to produce a draft that will cause
sufficient combustion and carry off the resulting smoke; incidentally
it assists ventilation. Many unsatisfactory heating plants and much
excessive fuel consumption are due to improperly constructed chimneys,
which are the rule rather than the exception. Although many of these are
more inefficient than dangerous, yet reports of the National Board of
Fire Underwriters[1] show that a larger number of fires are caused by
defective chimney construction than by anything else. The annual loss
resulting from such fires is greater than the fire loss from any other
cause. Poor chimney construction is responsible for smoke pollution of
the air, waste of fuel, and poor heating.

[1] "Dwelling Houses," a publication issued by the National Board of
Fire Underwriters in the interest of fire protection, has been used as
a basis for the matter relating to the requirements and construction of
chimneys and methods of fire protection.

The most common faults in chimney construction are:

1. The use of unsuitable materials. Clay sewer pipe, hollow building
blocks, or unprotected concrete should not be used.

2. Improper laying of brick. Brick should not be laid on edge and should
be properly bonded. Lining should be used in all brick chimneys the walls
of which are less than 8 inches thick. Lack of mortar, especially in the
perpendicular joints, ruins many an otherwise good chimney.

3. Failure to support the chimney properly. It should never be carried
on any timber construction of the building, and when it rests upon the
ground sufficient masonry foundation should be provided to prevent

4. Building inflammable material into the chimney or against it without
proper insulation.

5. Failure to anchor the smoke pipe properly to the chimney.

6. Neglect of the connection between smoke pipe and flue or of the flue
itself. The connection should be tight; rusted pipe should be replaced;
the chimney should be kept clean and the joints in the brickwork properly

7. Lack of a tight flue. A flue free from leakage is unusual. Every
flue should be tight enough to prevent escape of smoke when tested as
described on page 14. A leaky flue is the most frequent cause of heating
troubles, high fuel bills, and destructive fires.

8. Failure to maintain the full sectional area at the bend when a flue is

9. Use of the main heating apparatus flue for water heater or other
auxiliary equipment. The furnace or heater should have a separate flue.

10. Failure to provide a separate tight cleanout for each flue. Two or
more otherwise good flues may be rendered inefficient if led into one
cleanout, since air may be drawn from one into another and the draft in
all affected.

11. Presence of deep pockets leading to cleanouts. They may cause eddying
currents that are detrimental. Pockets should be only deep enough to
permit installing a cast-iron cleanout frame and door just below the
smoke pipe entrance. Deep pockets allow soot accumulation that may take


The draft depends entirely upon the chimney flue. The better the flue
the more satisfactory and efficient will be the operation of the entire
heating apparatus. The strength or intensity of the draft is dependent
mainly upon the tightness, size, and height of the chimney flue. The most
common error in chimney construction is failure to distinguish between
the size of flue necessary 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, yet have an area too small
to carry properly the volume of smoke. On the other hand, the size may be
sufficient but the chimney too low to produce a draft strong enough to
pull the air through the fire at a sufficiently rapid rate. Either fault
or a combination of the two will result in unsatisfactory service.

Draft in a chimney flue is caused by the difference in weight between
a volume of air on the outside and an equal volume of products of
combustion from the fire on the inside. The higher the temperature of
a given weight of air, the greater is its total volume and the lighter
the weight of its unit volume. This produces a condition of unbalanced
pressures at the base of the flue. The rising of the lighter gases within
the chimney tends to equalize the pressures. So long as the fire burns
this condition of unbalanced pressure persists, the result being draft.

This is the basic principle which governs chimney action and upon which
the draft depends. The greater the difference between the temperature in
the flue and that outside the greater the tendency toward equalization of
pressure and hence the better the draft. In summer the draft of a chimney
is not as good as in winter because the difference in temperature between
the outside air and that of the gases in the flue is less.

[Illustration: Round. Elliptical. Square. Oblong.

Fig. 1.--Round flues offer the least resistance to the passage of gases,
but most residence flues are made either square or oblong for structural


The most efficient chimney is one built perfectly straight with a round
or nearly round flue and a smooth interior surface. There is no advantage
in reducing the sectional area toward the top. The cross section and
height are determining factors. The transverse area must be sufficient to
pass the volume of air required to burn the fuel properly, and the height
must be great enough to insure against interference with the draft by
adjoining buildings or projections of the same building and to produce a
sufficiently strong draft.

Loss in draft strength is due to air leakage, and friction of the gases
against the sides of the chimney. A round flue (see fig. 1) is the most
desirable because it offers less resistance to the spirally ascending
column of smoke and gases. The elliptical is second choice so far as the
movement of the gases is concerned, but the difficulties that it presents
in manufacture and construction eliminate this shape. A rectangular
chimney either square or oblong is not effective over its full transverse
area; for the rising column, being approximately circular in section,
does not fill the corners. However, square or oblong forms are far
more common than the round, owing to the greater cost of round flue
construction. Square flues are preferable to oblong so far as efficiency
is concerned, but in the larger sizes of house flues the oblong shape is
more generally used because it fits to better advantage into the plan
of the house. An oblong flue should never have the long side more than
4 inches greater than the short side. A flue 8 inches by 16 inches is
bad flue construction for draft purposes. The sizes given in Table 1 are
recommended by the National Warm Air Heating and Ventilating Association.
Like all data for both high and low pressure flues, these sizes are based
on experience, not on scientific data, and are subject to modification by
further research. The dimensions given are for unlined flues. The actual
inside dimensions of flue tile are slightly different because of the lack
of standardization. In selecting the flue for a furnace or other large
heating unit an 8-inch by 12-inch size should be considered the minimum
for a lined or unlined flue, and 12 inches by 12 inches the minimum for
a lined or unlined flue whose height is more than 35 feet measured above
the grate level. If the chimney is designed for a small unit such as a
laundry stove or kitchen range an 8-inch by 8-inch flue may be used.

[Illustration: Fig. 2.--Top of chimney should be at least 2 feet above
the top of ridge in order that the wind currents may not be deflected
down the chimney.]

The proper size of flue depends upon the size of the heater or furnace
for which is to be used. All manufacturers' catalogues contain the size
of the smoke pipe for each particular heater, and from Table 1 (minimum)
dimensions for round, square, and oblong flues may be selected; or if the
catalogue contains stack sizes select the proper one. The flue tile to be
used should have a transverse net inside area approximately equal to that
of the smoke pipe.

  Table 1.

  Diameter                         Diameter
  of smoke                         of smoke
  pipe or              Height of   pipe or             Height of
  round       Size of  chimney     round     Size of   chimney
  chimney     chimney  flue above  chimney   chimney   flue above
  flue.       flue.    grate.      flue.     flue.     grate.
  Inches.     Inches.   Feet.      Inches.   Inches.     Feet.

     8        8 by 12     35         15      16 by 16     45
     9        8 by 12     35         16      16 by 18     45
    10       12 by 12     35         17      16 by 20     50
    11       12 by 12     40         18      16 by 20     55
    12       12 by 12     40         19      20 by 20     55
    13       12 by 16     40         20      20 by 24     60
    14       12 by 16     45


In Table 1 the minimum height of the chimney above the grate is given
as 35 feet. Higher chimneys are considered more satisfactory, and
authorities claim that any flue under 40 feet in height will produce
an erratic draft, good on some days but poor on others The force or
direction of the wind may be the cause, or the amount of moisture in
the air, or the quality of the fuel may be responsible. The higher the
chimney the less will be the possibility of counter air currents and
the stronger and more constant the draft. Soft coal and the sizes of
hard coal known as pea and buckwheat are apt to cake and fill up the air
spaces through the bed of the fire, with the result that an intense draft
is required to give the fuel sufficient air.

The top of the chimney should extend at least 3 feet above flat roofs and
2 feet above the ridge of peak roofs (see figs. 2 and 3), and it should
not be on the side of the house adjacent to a large tree or a structure
higher than itself (see fig. 4), for these may Cause eddies and force air
down the chimney. A poor draft will most likely result when the wind is
blowing in the direction indicated.

[Illustration: Fig. 3.--Extensions to the chimney required In order that
it might draw properly.]


Although chimneys are built unlined to save expense, those properly
lined with tile are undoubtedly more efficient. Linings prevent
disintegration of mortar and bricks through the action of flue gases.
This disintegration and that occurring from changes in temperature result
frequently in open cracks in the flue (see fig. 5-B) which reduce or
check the draft. If loose brick and mortar should fall within they may
lodge so as to cause partial or almost complete stoppage (see fig. 5-D).
The danger of this latter condition is greater if the flue be built
with offsets or bends. Any change in direction should be made as gradual
as possible and with an angle not greater than 30 degrees with the

The most important requirement for a flue lining is that it withstand
high temperatures and not be subject to disintegration by ordinary flue
gases. It should be made of fire clay and for the purpose. The thickness
should be 1 inch. It should be set in cement mortar with the joints
struck smooth on the inside. Each length of flue lining should be placed
in position, and the brick should then be laid around it; if the lining
is slipped down after several courses of brick have been laid, the joints
can not properly be filled with mortar and leakage is almost sure to

[Illustration: Fig. 4.--Large trees located near chimney tops may deflect
wind currents down the chimney. This may be avoided by placing the
chimney on the opposite side of the building.]

Well-burned clay flue linings are generally satisfactory for
dwelling-house chimneys used for stoves, ranges, fireplaces, and
furnaces. In regions where the fuel is natural gas, hot flue gases are
said to have caused linings to disintegrate and crumble off. In such a
case it may be necessary to use a fire clay that has stood the test or
line the chimney with fire brick.

Linings are manufactured in round, square, and oblong shapes, but not
in elliptical. The oblong and square shapes are better adapted to brick
construction than the round. They permit of simpler and less expensive
masonry work. On the other hand, the round shape produces better draft
and is easier to clean.

A fireplace flue, if straight, should be lined from the throat
continuously to the top. The smoke chamber should be lined with fire
clay or cement mortar one-half inch thick. In case the masonry in front
of the throat is less than 8 inches thick the lining should start at the
bottom of the lintel. The hottest part of the flue is at its throat, and
if it is not lined at that point or if the masonry is not of sufficient
thickness, there is danger of overheating. Careful attention should be
given to details of flue construction in order to assure satisfactory
operation and reduce the fire hazard.


The best location for the chimney is near the center of the building, for
when so located its four walls are kept warm; cold winds can not chill
it and cause it to draw poorly. However, it is not always possible to
plan the arrangement of rooms so that the chimney may be thus located.
The outside wall of a chimney should be at least 8 inches thick in order
to reduce heat loss and the chance of air leakage into the flue.

[Illustration: Fig. 5.--A. An unlined chimney before use. B. Same
chimney, after being in service. Frequently the heat and weather cause
the mortar to disintegrate so that air leaks in through the joints,
causing a reduction in the draft. C. Same chimney as A, showing terra
cotta flue lining in place. D. An unlined chimney with offset. Loose
brick and mortar may fall and become lodged at the offset during
construction or loosening of the points and disintegration may cause
bricks from an uncapped chimney to check the draft completely.]

If the flue is lined and the chimney is not higher than 30 feet, its
walls, if of brick, may be made 4 inches thick, provided adjacent
inflammable material is properly insulated. If unlined, the walls should
not be less than 8 inches thick. It is not good practice to place the
linings of two flues side by side. If there is more than one flue in a
chimney, the flues should be separated from each other by a division
wall of brick at least 4 inches thick (see fig. 6), bonded into the side
walls, and the joints of the flue linings should be staggered or offset
at least 6 inches (see fig. 7). This construction insures stability,
reduces the chance for air leakage between flues, and prevents the
possibility of a fire in one flue involving an adjacent flue. If stone
is used in chimney construction, the walls should be at least 4 inches
thicker than brick walls.

Walls of concrete chimneys should be not less than 4 inches thick or
else they should be reinforced in both directions; otherwise cracking
during the setting of the concrete or, later, due to temperature changes
or unequal settlement of the foundation is apt to occur. Concrete blocks
are not recommended, but if they are used each block should be reinforced
with steel running continuously around it and the blocks should be not
less than 4 inches thick. They should be lined with the best flue lining.
All monolithic concrete chimneys with walls less than 8 inches thick
should be lined.


It is not unusual to find an opening into a chimney other than for the
smoke pipe of the main heating apparatus. This is a frequent cause of
unsatisfactory operation. No range, stove, fireplace, or ventilating
register should be connected with the chimney flue built for the heating
apparatus. If it should be desired to use an existing abandoned fireplace
chimney for a range or stove the fireplace flue should be closed tight
about a foot below the place where the smoke pipe enters.

[Illustration: Fig. 6.--A division wall of at least 4 inches of brick
should separate each flue from any others in the same chimney. Either of
the arrangements shown will produce a good bond.]

There should be but one connection with a flue, if for no other reason
than to decrease the fire hazard. Fires frequently occur from sparks
that pass into the flue through one opening and out through another. Two
stoves, one on the first floor and one on the second, may be connected
with the same chimney flue, but if the fire in the upper stove is hotter
than in the lower, the lower will have practically no draft.

A soot pocket provided with a door for cleaning it out is very
convenient. The door should be placed just below the smoke pipe opening,
and care must be taken to see that it fits snugly and is always closed so
tight that no air can get in.


All chimneys should be built from the ground up. None of the weight
should be carried by any part of the building except the foundation.
Proper foundations should be provided at least 12 inches wider all round
than the chimney. If the chimney is an exterior one, and there is no
basement or cellar, its foundation should be started well below the frost
line. Otherwise the base of the chimney should be at the same level as
the bottom of the foundation of the building.

No chimney should rest upon or be carried by wooden floors, beams, or
brackets, nor should it be hung from wooden rafters. Wood construction
shrinks, and beams supporting heavy loads always deflect in time. Sagging
of the beams injures the walls and ceilings of the house and is apt
to crack the chimney and render it dangerous. Chimneys usually extend
several feet above the roof, exposing considerable surface to the wind,
and unless the support is stable they are likely to sway during a gale
with the possibility of the joints at the roof-line opening. Openings in
a flue at this point are especially dangerous, for sparks from the flue
may come into contact with the woodwork of the roof. This swaying may
also cause leaks in the roof.

[Illustration: Fig. 7.--Chimney and roof connection. Sheet metal A should
have shingles K over it at least 4 inches. Apron B bent as at E with base
flashings C, D, and H and cap flashings P and G, lapping over the base
flashings provide watertight construction. When the chimney contains two
flues the joints should be separated as shown.]

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. It is well to use cement mortar for the entire chimney
construction. All mortar used for chimney construction, except for laying
firebrick, should be proportioned as follows: Two bags of Portland
cement, not less than 188 pounds, and one bag of dry hydrated lime, 50
pounds, thoroughly mixed dry, and to this mixture should be added three
times its volume of clean sand with sufficient water to produce proper
consistency. When dry hydrated lime is not available, 1 cubic foot of
completely slaked lime putty may be substituted for the dry hydrate.


Brick chimneys should be capped with stone, concrete, or cast-iron.
Unless a chimney is capped the top courses of brick may become loosened
and therefore dangerous. Plain topped chimneys will last longer and are
safer than those of an ornamental character. The opening in the cap piece
should be the full size of the flue.


Where the chimney passes through the roof the construction should provide
space for expansion due to temperature changes, settlement, or slight
movement of the chimney during heavy winds. (See fig. 7.) Copper is the
best material for flashings. It is easier to handle than galvanized
sheet-metal, which is more often used because of its lesser cost, but
which will corrode in time, both from inside and outside exposure. Tin or
black iron are cheaper but will rust quickly unless frequently painted.
Lead and zinc are expensive and should not be used for chimney flashings,
for in case of fire under the roof they will melt and leave an opening to
create a draft by which the intensity of the fire will be increased.

[Illustration: Fig. 8.--A. Wrong connection, producing interference and a
poor draft. B. Correct construction, producing a good draft by providing
a free passage for the gases.]


Proper care in setting and looking after smoke pipes connecting with
chimneys would greatly lessen the number of fires chargeable to defective

In fitting the smoke pipe no opening should be left around it, and the
pipe should not project into the flue lining. (See fig. 8.) The joint
should be made air-tight by a closely fitting collar and boiler putty or
fireproof cement. The proper construction is shown in figure 8-B, but
if the pipe extends into the flue a shelf is formed on which soot will
accumulate, the flue area will be reduced and a poor draft may result.

Smoke pipes should enter the chimney horizontally, and the connection
through the chimney wall to the flue should be made with fire clay or
metal thimbles securely and tightly set in the masonry. If the walls are
furred, no wood should be within 12 inches of thimbles or any part of
the smoke pipe. The space between the thimble and wood furring should be
covered with metal lath and plaster.

[Illustration: Fig. 9.--Smoke pipe passing through a partition. A,
7/8-inch sides of partition; B, 2 by 4 studs in partition; C, ventilating
holes in the double galvanized iron ventilating thimble D. Thimble should
be at least 12 inches larger than pipe S.]

Flue holes when not in use should be closed with tight fitting metal
covers. If the room is papered the metal covers may also be papered,
provided there is no other smoke connection with the flue, or provided
a protective coating of asbestos paper is first applied over the metal.
If there is another connection the metal may become hot enough to scorch
the unprotected wall paper or set it afire. No smoke pipe should be
permitted within 18 inches of any woodwork unless at least that half of
the pipe nearest the woodwork is protected properly by 1 inch or more of
fireproof covering. A metal casing 2 inches from the upper half of the
pipe is sometimes employed to protect woodwork directly above it. When a
smoke pipe is so protected it should never be less than 9 inches from any
woodwork or combustible material. The storage of wooden boxes, barrels,
or any combustible should not be permitted under or near a furnace smoke

If a smoke pipe must be carried through a wood partition the woodwork
should be properly protected. This can be done by cutting an opening in
the partition and inserting a galvanized iron double-walled ventilating
thimble at least 12 inches larger than the smoke pipe (see fig. 9), or
protection may be afforded by at least 4 inches of brickwork or other
incombustible material. Smoke pipes should not pass through floors,
closets, or concealed spaces. They should not enter a chimney in a
garret. They should be cleaned at least once a year.


All wooden construction adjacent to chimneys should be insulated. A
space of 2 inches should be left between the outside face of a chimney
and all wooden beams or joists. This space should be filled with some
porous, nonmetallic, incombustible material. Loose cinders serve well.
(See fig. 10.) Do not use brickwork, mortar, or solid concrete. The
filling should be done before the floor is laid, as it not only forms
a fire stop but prevents accumulation of shavings or other combustible
material. Baseboards fastened to plaster which is directly in contact
with the outside wall of a chimney should be protected by placing a layer
of fireproof material at least one-eighth inch thick between the woodwork
and the plaster. (See fig. 10.)

[Illustration: Fig. 10.--No woodwork should be permitted closer than 2
inches to the outside face of a chimney. Baseboards in front of chimneys
should be protected with asbestos board.]

Wooden studding, furring, or lathing should not under any circumstances
be placed against a chimney. Wooden construction should be set back
from the chimney as indicated in figures 11 and 12; or the plaster may
be applied directly to the masonry or to metal lathing laid over the
masonry. The former is the better method, as settlement of the chimney
will not crack the plaster. It is recommended that a coat of cement
plaster be applied directly upon the masonry of any parts of a chimney
that are to be incased by a wooden partition or other combustible

[Illustration: Fig. 11.--No wooden studding, furring, or lathing should
be placed against the chimney. It should be set back as indicated in this
figure and in fig. 12.]

[Illustration: Fig. 12.]


Every flue should be subjected to a smoke test before the heater is
connected with it. This may be done as follows: Build a paper, straw,
wood, or tar-paper fire at the base of the flue, and when the smoke is
passing in a dense column tightly block the outlet at the top by laying a
wet blanket over it. If leakage exists at any point, it will immediately
become apparent by the appearance of smoke at the opening. Flues so
tested frequently reveal very bad leaks into adjoining flues or directly
through the walls or between the linings and the wall. When the smoke
test indicates leakage, the defect should be remedied before the chimney
is accepted for use. Remedying such defects is usually difficult, hence
it is wise to watch the construction closely as it progresses. Many brick
masons say that all flues leak. This is not true; every flue should be


If a smoke test shows no leakage and the flue is straight, a hand mirror
held at the proper angle at the base affords a means of examination for
obstructions. Usual causes of stoppage are broken tile leaning inward,
mortar accumulations, loose bricks, bird's nests, partly burned paper,
soot from soft coal, tarry deposits from burning wood, etc. A weighted
bag of hay or straw attached to the end of a rope may be passed up and
down the flue to clean it if there is not too great an offset in it.


The use of the fireplace is a very old method of house heating. As
ordinarily constructed fireplaces are not efficient and economical. The
only warming effect is produced by the heat given off by radiation from
the back, sides, and hearth of the fireplace. Practically no heating
effect is produced by convection; that is, by air currents. The air
passes through the fire, is heated, and passes up the chimney, carrying
with it the heat required to raise its temperature from that at which it
entered the room and at the same time drawing into the room outside air
of a lower temperature. The effect of the cold air thus brought into the
room is particularly noticeable in parts of the room farthest from the

The open fireplace, however, has its place as an auxiliary to the
heating plant and for the hominess that a burning fire imparts to the
room. If one is to be provided, the essentials of construction should be
understood and followed so that it will not smoke.


In order that satisfactory results may be obtained from an open
fireplace, it is essential: First, that the flue have the proper area;
second, that the throat be correctly proportioned and located; third,
that a properly-constructed smoke shelf and chamber be provided; fourth,
that the chimney be carried high enough to avoid interference; and fifth,
that the shape of the fireplace be such as to direct a maximum amount of
radiated heat into the room.


The sectional area of the flue bears a direct relation to the area of the
fireplace opening. The area of lined flues should be a tenth or more of
that of the fireplace opening. If the flues are unlined the proportion
should be increased slightly because of greater friction. Thirteen square
inches of area for the chimney flue to every square foot of fireplace
opening is a good rule to follow. For the fireplace shown in figure 13-A,
the opening of which has an area of 8.25 square feet, there is required
a flue having an area of 107 square inches. If this flue were built of
brick and unlined it would probably be made 8 inches by 16 inches, or 128
square inches, because brickwork can be laid to better advantage when the
dimensions of the flue are multiples of 4 inches. If the flue is lined
the lining should have an inside area approximating 107 square inches. It
is seldom possible to secure lining having the exact required area, but
the clear area should never be less than that prescribed above.

[Illustration: Fig. 13.--A. Top of throat damper is at DD, smoke shelf
at CO. Side wall should not be drawn in until the height DD is passed.
This assures full area. If the drawing in is done as indicated by lines
EF and EG, the width of the throat becomes less than the width of the
opening and causes the air currents to pile up in the corners of the
throat, resulting frequently in a smoky fireplace. B. Correct fireplace

Failure to provide a chimney flue of sufficient sectional area is in many
instances the cause of an unsatisfactory fireplace. The cross section
should be the same throughout the entire length of the chimney. Do not
contract the flue at the chimney top, for that would nullify the larger
opening below; if it is necessary to change the direction of a flue the
full area should be preserved through all turns and bends, and the change
should be made as gradual as possible.


In figure 13-B is shown the throat, the narrow opening between the
fireplace and the smoke chamber. Correct throat construction contributes
more to efficiency than any other feature except proper flue design.
A flue twice as large as is necessary brought straight down to the
fireplace without constriction at the throat would result in a poor
draft, for the draft does not depend upon the largeness of the flue but
upon its proper proportioning to the fireplace and throat. The arrows
indicate the upward flowing currents of warm air which are thrown forward
at the throat and pass through the smoke chamber into the flue on the
inner side. This rapid upward passage of air causes a down current on the
opposite side, as indicated by the descending arrows. The down current
is not nearly as strong as the up current, but it may be of such force
that if there be no throat to the fireplace (see fig. 14) to increase the
velocity of the upward current by constricting it, the meeting of the
two currents will result in smoke being forced out into the room. Thus
it frequently happens that a fireplace has an ample flue area and yet
smokes badly. The influence of the throat upon the upward and downward
air currents is shown in figure 13-B.

[Illustration: Fig. 14.--Fireplaces constructed like this without throat
will very likely smoke.]

The area of the throat should not be less than that of the flue. Its
length should always be equal to the width of the fireplace opening.
(See fig. 13-A.) The sides of the fireplace should be vertical until the
throat is passed. (DD in fig. 13-A.) Above the throat the sides should
be drawn in until the desired flue area is attained. The throat should
be set 8 inches above the location of the lintel, as shown in figure 13,
A and B. The wrong way to place the throat damper is shown in figure
15. The throat should not be more than 4 or 5 inches wide. The lesser
width is a safe standard. If a damper is installed the width of the
brick opening at the throat will depend upon the width of the frame of
the damper, the width of the throat proper being regulated by the hinged
cover of the damper. If the throat damper is omitted the opening should
be 4 inches, as shown in figure 16. The smoke shelf should not be bricked
up but should conform to the dotted lines. The depth of the smoke shelf
should be the same for a 2-foot as for a 10-foot fireplace opening.

Proper throat construction is so necessary to a successful fireplace that
the work should be carefully watched to see that the width is not made
more than 4 inches and that the side walls are carried up perpendicularly
until the throat is passed, so that the full length of opening is
provided. All masons do not appreciate these fine but necessary points.
Many prefer their own and sometimes will ignore the proper methods. It
is therefore advisable to inspect the work several times a day as it
progresses and thus avoid poor results. When trouble is experienced in an
existing fireplace that has ample flue area, it is usually found that the
formation of the throat is the cause.

[Illustration: Fig. 15.--Wrong location for throat damper. The throat is
so low that the accumulation of gases at the point constricted weakens
rather than improves the draft with greater likelihood of a smoky
fireplace. Note that the smoke shelf is bricked up. This is wrong.]


A smoke shelf and chamber are absolutely essential. The shelf is formed
by setting the brickwork back at the top of the throat to the line of the
flue wall. The shelf should be the full length of the throat. The depth
of the shelf should be not less than 4 inches. It may vary from this to
12 or more, depending upon the depth of the fireplace.

The purpose of the smoke shelf is to change the direction of the down
draft so that the hot gases at the throat will strike it approximately
at a right angle instead of head on. Therefore the shelf should not be
bricked up as shown in figures 15 and 16, but should be made as wide as
the construction will permit at a height of 8 inches above the top of the
fireplace opening.

The smoke chamber is the space extending from the top of the throat up to
the bottom of the flue proper and between the side walls, which may be
drawn in after the top of the throat is passed. The area at the bottom
of the chamber is quite large, since its width includes that of the
throat added to the depth of the smoke shelf. This space is capable of
holding accumulated smoke temporarily in case a gust of wind across the
top of the chimney momentarily cuts off the draft. Smoke might be forced
into the room if there were no reservoir to hold it. The smoke chamber
also lessens the force of the down draft by increasing the area through
which it passes. If the walls are drawn inward 1 foot for each 18 inches
of rise, friction is reduced and interference with the draft lessened.
The walls should be smooth inside, for roughness seriously impedes the
upward movement of the air currents.


The shape of the fireplace proper should be as indicated in figure 13-A.
The back should pitch forward from a point a little less than half way
from the hearth to the top of the opening, and the sides should be
beveled as indicated. Straight back and sides do not radiate as much heat
into the room.

[Illustration: Fig. 16.--This construction without a throat damper
directs the down draft so that it meets the up draft almost at the
throat, which is more faulty than the construction shown in fig. 15, for
there the lid of the damper deflects the down current.]


A properly designed throat damper affords a means of regulating the fire.
The damper consists of a cast-iron frame with a lid hinged preferably
at the back so that the width of the throat opening may be varied from
nothing to 6 inches. There are a number of patterns on the market, some
of which are designed to support the masonry over the fireplace opening.

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

In houses heated by furnaces or other modern systems fireplaces without
throat dampers interfere with even heating, particularly in very cold
weather. An open fire must be supplied with air and the larger the fire
the greater the quantity required; a fireplace with a width of 5 feet
or more may pull air from distant parts of the house. This air that
is heated at the expenditure of fuel in the furnace is carried up the
chimney and wasted, but with a throat damper open only 1 or 2 inches a
slow fire of hardwood can be kept going without smoking the room, thus
reducing materially the waste of hot air.

[Illustration: Fig. 17.--Smoke dampers with lids hinged in the center do
not turn the up draft as well as do those hinged at the rear side.]


The throat damper should be as wide as the fireplace, so the side walls
should not be drawn in until after the throat is passed. Smoke dampers
with lid hinged at the back will help the smoke shelf to turn the down
draft; if the lid is hinged in the center the downward and upward
currents are apt to conflict. The placing of the damper varies with the
type, but generally the bottom of the frame is built into the brickwork
at the level of the top of the fireplace opening, forming the throat and
supporting the masonry above it.


Pleasing proportions in the fireplace opening are desirable. The width
should generally be greater than the height, but as 30 inches is about
the minimum height consistent with convenience in tending the fire,
a narrow opening may be made square. Three feet and a half is a good
maximum for height of opening unless the fireplace is over 6 feet wide.
The higher the opening the greater the chance of a smoky fireplace.

A fireplace should be in harmony with the rest of the room in proportions
and details. This consideration and the kind of fuel to be used largely
determine the size of opening.

Generally speaking the day of large farmhouse fireplaces capable of
receiving cordwood is past. The tending of fires usually falls to the
housewife, and cordwood is a heavier weight than she should handle and
can not be stored near at hand. Cordwood cut in two is easily handled; so
that a 30-inch width is about the minimum for farmhouses where wood is
used for fuel. If coal is burned the opening may be made narrower.


Unless a fireplace with a 6-foot opening is made fully 28 inches deep, in
order that large logs will lie well inside, the advantage of the wide
opening is lost, for the logs will have to be split. A shallow opening
throws out more heat than a deep one of the same width, but can take only
sticks of smaller diameter; thus it becomes a question of preference
between the greater depth which permits of large logs that burn longer
and require less frequent replenishing and the shallower which takes
lighter sticks and throws 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 18
inches is advised, to lessen the danger of brands falling out on the
floor. Wire guards should be placed in front of all fireplaces. In
general, the wider the opening the greater should be the depth.


The hearth should be flush with the floor, for sweepings may then be
brushed into the fireplace. An ash dump located in the hearth near the
back of the fireplace is convenient for clearing ashes and other refuse
from the hearth provided there is space below for an ash pit. The dump
consists of a cast-iron metal frame, with pivoted cover, through which
the refuse can be brushed into the ash pit below. The ash pit should be
of perfectly tight masonry and provided with a tightly fitting cleanout
door. If a warm-air flue, as described on page 27, is provided, the ash
dump will have to be located near one side of the hearth instead of in
the center.


The jambs of the fireplace should be of sufficient width to give
stability to the structure both actually and in appearance. For a
fireplace opening 3 feet wide or less, 16 inches is generally sufficient;
for wider openings similar proportions should be kept. Greater widths may
be required to harmonize with the proportions of the rooms, and the above
should be taken as a minimum.


The back and sides of the fireplace should be constructed of firebrick
only. The bricks should be laid flat with the long sides exposed, for if
placed with the face exposed there is danger of their falling out.


In small fireplaces sagging of the arch over the opening seldom occurs,
but in fireplaces over 4 feet wide it is not uncommon. It is due to
insufficient support of the masonry. Except in massive construction
there generally is not sufficient masonry at the sides of the opening to
resist the thrust of arch construction; hence it is usual to support
the masonry with iron, which, if too light, will sag. Too small an iron
will become so hot that its tensile strength is lowered until it bends. A
heavy flat bar at least one-half inch thick is sometimes used or a T-bar
which has greater strength, but less metal; the wider the opening the
heavier the bar required.


A number of patents have been obtained for improvements in fireplace
heating. Most of them, depending on the fact that hot air rises, deliver
air heated in or around the fireplace through a register, located above
the fire, into the upper part of the room, which is always the warmest
part. Furthermore, they require a specially built chimney, precluding
the installation of such a device in an existing fireplace. Unless fresh
outside air is supplied there is no improvement in the warming of the

Patent No. 1251916, issued to Joseph Parsons, of Lakeville, Conn., and by
him assigned to the United States Government, presents means of greatly
increasing the efficiency of fireplace heating. The inventor's claim
differs from other claims for improving fireplace heating in that the
operation of his device depends upon the suction created in the chimney
by the hot air rising from the fireplace and therefore makes possible the
delivery of heated air through a register located at any place in the
room or at the hearth. Furthermore, it permits of installation of one of
the simpler types in an existing chimney.

For a fire to burn it must be supplied with oxygen. If a fire were built
in a fireplace in an air-tight room it would go out as soon as the oxygen
present had been consumed unless a down draft in the chimney supplied
the needed air. As our fireplace fires do not go out so long as they are
fed with fuel it is obvious that the required air supply is obtained
from somewhere. Any one who has depended upon a fireplace to heat a room
knows that the part of the room farthest from the fire is the coldest and
that the temperature around the windows is especially low. In fact the
harder the fire burns the colder it is at the windows. The fire must have
air, and as cracks exist around windows and doors the air enters through
them. The volume entering is equal to that passing up the chimney. This
air comes from outside at a low temperature. Figure 18 illustrates how a
fireplace fire supplies its needs. When it grows colder outside a bigger
fire is made. The bigger the blaze the greater the quantity of outside
air drawn into the room through every crack and crevice until, when the
outside temperature gets below the freezing point, there is no comfort in
the room beyond the immediate vicinity of the fire.

[Illustration: Fig. 18.--All air required for feeding the fire must pass
through the room, entering through cracks around windows and doors and
producing an uncomfortable temperature in all parts of the room except
near the hearth.]

If a room were so tight that the air leakage were insufficient to supply
a fireplace fire, it would not burn properly and would smoke. If a pane
of glass were removed from a window cold air would rush in through the
opening. If the glass were replaced and an opening of equal area be made
through the chimney, as shown in figures 19, A and B, so that air could
be admitted into the room as indicated by the arrows in the plan, figure
19-B, an equal volume of cold air would be drawn through this opening.
As it comes into contact with the metal form the air becomes heated, so
that when delivered into the room its temperature would be 100 degrees or
higher, depending upon the radiating surface of the hearth, assuming an
outside temperature of 32 degrees. (Tests by the writer have shown this
temperature to be higher than 125 degrees.) If the chimney opening be
closed and the pane of glass be again removed the temperature of the air
entering through the window would be 32 degrees. It is obvious that the
room will be more effectually heated when the air required for combustion
is supplied at a high temperature than when supplied through cracks and
crevices at a low temperature. All our homes should be made fairly tight
for greater comfort in winter. In such a house, with doors and windows
closed, the suction caused by the fire can thus be utilized to draw into
the room outside air heated in passing through a metal flue on which the
fire is burning.

The principle may be applied in various forms. Figure 19-A illustrates
a simple form for use in connection with an outside chimney. A piece
of galvanized sheet iron is bent to the proper form and set into the
fireplace so as to leave an air space between it and the back and sides
of the fireplace. An opening to the outside is made by removing two
or three courses of brick. Air enters through this, becomes heated by
contact with the metal, and is delivered into the room at the sides of
the fireplace, as indicated in the plan of figure 19-B. It immediately
rises within the room, gives up part of its heat, and eventually whirls
about and into the fire, as indicated by the arrows in figure 19-A. This
form would not necessarily heat the entire room effectually; it would,
however, supply heated air for the fire in volume sufficient to replace
or materially reduce the quantity of cold air which would otherwise enter
through window and door cracks. With a brisk fire burning, a rush of warm
air can be felt 6 or 8 feet away from the fireplace.

[Illustration: PERSPECTIVE]

[Illustration: Fig. 19.--Simple form of warm-air flue for outside
chimney. Air required for feeding the fire is brought in from the outside
around a metal form set in the fireplace, with a space between it and
the back and sides of the brickwork. As the cold outside air passes
around the metal it becomes heated and is delivered into the room at a
temperature much higher than where it is pulled in through window and
door cracks. The result is a much more comfortable room.]

This simple form may be built as follows: A piece of roofing tin about 6
inches wider than the height of the fireplace opening, with length equal
to the width of the opening plus twice the depth of the side, should be
secured. It should then be marked and cut as indicated in the form (fig.
19-B), and bent into a shape similar to that shown in the perspective,
same figure. When placing it, there should be a space left between the
tin and the brickwork at both back and top. The back and sides at the top
should be bent back 2 inches to meet the brickwork. The crack or joint
should be tightly closed with asbestos or furnace cement. The tin form
rests on the 4-inch bottom flange. The joint here can be made tight by
placing a few brick on the flange and covering with ashes, or a metal
plate cut to the proper shape may be laid upon and preferably riveted
to the lower flanges of the back and sides. The form should be as high
as the opening and the metal sides should project about 3 inches beyond
the jambs, so as to throw the heated air well out into the room. A
one-fourth-inch rod placed across the top of the tin form directly under
the arch iron of the fireplace assists in holding the top of the tin
firmly against the brickwork.

[Illustration: Fig. 20.--Simple form of warm-air flue for inside

[Illustration: Fig. 21.--Improved form of warm-air flue for Inside
chimney. The increased radiating surface obtained by conducting the metal
flue up the back of the fireplace heats the air to a higher temperature
so that it is delivered into the room farther from the outlet duct.]

Figure 20 shows a simple form for use with an inside chimney. A hole may
be cut in the hearth on one side and connected with the outside by means
of a passage through the chimney foundation. The manner of providing
this passage will depend upon the construction in the particular case.
A galvanized sheet-metal box with a division plate extending part way
through it is set on the hearth. The side over the opening is bent
down in front, as at A, so that the entering cold air must pass to the
rear around the division plate and then out into the room in front of
the hearth, as at B. The fire, on top of the metal flue, heats the air
issuing at B as it flows under it. Figure 21 shows an improved form
in which the flue and division plate are extended up the back of the
fireplace. This presents considerably more radiating surface, so that
the air can be heated to a higher temperature. The air issuing from this
flue at B is discharged farther out into the room. If there is a cellar
under the floor a metal duct must be employed to bring fresh air from an
opening in the outside wall, just below the joists, to the hole in the
hearth. Cellar air should never be sucked through the flue. All openings
under the house or through the wall should be screened to keep out rats
and mice, and doors should be provided to close the openings entirely if

[Illustration: Fig. 22.--Improved form of warm-air flue with floor
register. This method increases the efficiency of fireplaces many times
by delivering the air that must be supplied to the fire into the room at
temperatures of 100° and higher, depending upon the form and extent of
the heating surface at the back of the fireplace, and delivering it to
the coldest part of the room so that heat is distributed more effectively
and the entrance of cold air around windows and doors is reduced to a

Figure 22 shows a more elaborate installation. This insures very
satisfactory heating with a fireplace fire. The piece A B C D of
galvanized metal has a rectangular cross section. Two or three courses of
brickwork are omitted and the metal duct is set into the fireplace, so
that radiation from the fire impinges upon its surface from B to D. The
air entering from outside at AE is heated as it passes through the flue
behind and under the fire and is carried through another rectangular duct
under the floor to a register located in a far part of the room. Out of
this register air in large volume is discharged at a high temperature.
This air heats the far part of the room and other parts as it travels
from the register upward and through the room to the fireplace. Thus the
fireplace heats the room by convection of heat as well as by radiation,
and all parts of the room are more comfortable than if radiation alone
were depended upon. A test of an installation similar to that shown in
figure 22 was made by the writer. The fireplace and suction flue were
built in a cabin measuring 24 feet square by 9 feet high. The test was
conducted late in November on a night when the outside temperature was
24° F. It was the first fire built in the fireplace in that season,
consequently all the materials of the building were cold. The room was
practically air-tight; very little leakage could be felt around the
windows. A temperature of slightly over 100° was recorded directly over
the register, in the center of the room it was 72°, and in the farthest
corner a thermometer, hung about 18 inches from the wall between two
windows, showed 65°.

Thus the efficiency of fireplaces may be materially increased, the degree
depending upon the character of the air duct installed. Even in the
simple types the air required to make the fire burn enters the room at a
higher temperature at the floor instead of around windows and doors at
a low temperature; windows and doors may therefore be made tight, so as
to reduce the cold-air leakage. The type with a register in the far part
of the room supplies heat to parts of the room or to an adjoining room,
which would receive little heat if radiation only were relied upon. This
means of improving fireplace heating is particularly adapted to small
houses in the South, where the open fire is the most common method of
house heating. As the simple types require only galvanized Sheet metal
bent at right angles, it is within the means and ability of many to
supply themselves with flues of their own making.


Transcriber Note

Illustrations were moved so as not to split paragraphs. Hyphenization has
been standardized to the most common form.

*** End of this Doctrine Publishing Corporation Digital Book "Farmers' Bulletin 1230 - Chimneys & Fireplaces - They Contribute to the Health Comfort and Happiness of the - Farm Family - How to Build Them" ***

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