Mechanics of the Household. E. S. Keene
is too small for its requirements, the temperature of the air-heating surfaces must be very high to provide the desired amount of heat. Under such a condition the efficiency of the furnace would be low, since in all cases where rapid combustion is required the available amount of heat per pound of coal consumed is low. With a large amount of heating surface, the air remains in contact with the hot surface a relatively longer period and the desired temperature is reached with the expenditure of a smaller amount of fuel. A momentary exposure of the air to a red-hot surface is far less effective than a prolonged contact with a surface having only a moderate temperature. Time is an element of great importance in heating air. In considering the relative merits of two furnaces with the same amount of grate surface, that with the larger amount of heating surface will evidently be the most efficient.
The supply of heat comes primarily from the burning coal on the furnace grate. The grate surface should be large enough in area to permit the required quantity of heat to be generated by the burning fuel with a moderate fire. If the grate surface is too small for the required purpose, a hot fire will be necessary, when the normal amount of heat is demanded by the house. During extremely cold weather, particularly when accompanied by high wind, the extra heat demanded to keep the house at the desired temperature makes necessary the use of an amount of fuel that cannot be burned on the grate unless the fire is forced. Hot fires can be kept up only at the expense of a large amount of heat, and the resultant efficiency of the furnace is reduced.
High furnace temperatures are always attended by a large loss of heat. The vastly greater quantity of air necessary to create the combustion, the high temperature of the chimney gases and the increased velocity of the heated gases through the furnace, all tend to increase the amount of heat that is sent up the chimney, and to decrease the percentage of heat that is delivered by the furnace. In order to heat the house economically the furnace must be large enough to easily generate the required amount of heat demanded in the most severe weather.
Furnace-gas Leaks.
—The presence of furnace gas in the atmosphere of a house is not only annoying but may be a source of danger. Gas leaks are commonly due to the imperfect union of the various parts of which the furnace is composed.
Cast-iron furnaces are constructed in sections that are assembled to form a complete plant. In assembling, the various parts of contact must be carefully joined to prevent the gases in the fire-box from escaping into the air-heating space. In the manufacture of cast-iron furnaces it is practically impossible to form gas-tight joints by the contact of the metal alone. In the erection of the furnace all doubtful joints are filled with stove putty. Furnaces of good design require the use of the least amount of this material.
Stove putty is composed of finely divided graphitic carbon that is made into a paste suitable for filling all imperfect joints. When the putty hardens it withstands the heat to which it is subjected, without shrinking. In the course of time, however, the putty may be displaced and leave openings through which the furnace gases may leak into heating space and thus enter the house. Leaks of the kind may be stopped by renewing the putty which may be obtained from any dealer in stoves.
Location of the Furnace.
—The location of the furnace will generally be governed by the exposure of the house and the location of the chimney. In all exposed rooms on the windward side of the house the temperature will be lower and the air pressure higher than in other parts of the house. The increase in atmospheric pressure makes it necessary to supply to such rooms the hottest air practicable. The conducting pipes, therefore, should be most directly connected with the furnace and with the least run of horizontal pipe. The proper place for the furnace is as near as possible the coldest place of the house.
It is a common practice to place registers near the inner corner of the room, in order to economize in conducting pipe, in horizontal runs. A small amount of economy in first cost is thus secured but the efficiency of the apparatus is sacrificed.
The greatest objection to placing the registers and conducting pipes in the outer walls of buildings is that of loss of heat, due to exposure to the outside cold and the resulting loss in circulation. Losses of this kind may be prevented by covering the ducts with the necessary non-conducting material. The registers should occupy a place in the room nearest the entering cold air.
Fig. 46.—Method of conducting warm air from the furnace to the registers.
Flues.
—It is customary to place the conducting pipes for the first floor in such a way as to use only the shortest connections. The flues used for the second floor produce, as in a chimney, a greater velocity of flow to the air and as a consequence larger horizontal pipes are used at the furnace. All horizontal pipes should have upward slant, as much as the basement will permit.
The velocity of the air in the conducting flues will depend on two factors: the height of the flue, and the temperature of the air. To prevent the loss of the temperature of the air, the flue should be covered with at least two layers of asbestus paper bound with wire. Wall flues are commonly flattened and occupy a place in the wall between the studding. Each flue should have a damper at the furnace, that will permit the heat to be shut off from any part of the house.
Rules for proportioning of registers and conducting flues to suit rooms of various sizes are entirely empirical. The sizes of registers and flues found satisfactory in practice is generally a guide for the designer. The following table is taken from a manufacturer’s catalogue and gives a list of sizes that have proven satisfactory under a great variety of conditions and may be taken as good practice:
| First Floor | |||
|---|---|---|---|
| Sizes of registers in inches | Diameter of pipes in inches | Size of rooms in feet | Height of ceilings in feet |
| 12 by 15 | 12 | 18 by 20 | 11 |
| 10 by 14 | 10 | 15 by 15 | 10 |
| 9 by 12 | 9 | 14 by 15 | 9 |
| 8 by 12 | 9 | 13 by 13 | 9 |
| Second Floor | |||
| 10 by 14 | 10 | 18 by 20 | 10 |
| 9 by 12 | 9 | 16 by 16 | 9 |
| 8 by 12 | 8 | 13 by 13 | 8 |
| 8 by 10 | 7 | 12 by 12 | 8 |
The furnace is not only a means of heating the house but may be a means of ventilation as well; to this end it is desirable to arrange the air supply of the furnace to connect with the outside air. This arrangement assures a supply of oxygen even though no special means is arranged for discharging the vitiated air from the rooms.
Fig.