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
room.
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
chimney.]
[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
desired.
[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
minimum.]
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.