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.
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.
WASHINGTON: GOVERNMENT PRINTING OFFICE: 1921