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HEATING must always be considered in connection with ventilation. This is necessary for several reasons. The combustion of coal utilizes oxygen, and as a result the products of combustion are given off. In order to supply air for combustion and draught fresh air must be introduced. It is also necessary to have a supply of fresh air to replace the heated air which escapes from the building

Loss of Heat from Buildings.—Heat is required to warm the air of a room to a given temperature, to supply the loss of the heat from radiation and conduction from the windows and walls, and to supply the heat for the air required for ventilation. The amount of heat required for these various purposes will depend largely upon the construction of the building, the amount needed for purposes of ventilation, and the difference between the inside and outside temperature. The loss of heat from the walls of buildings depends upon the material used, its thickness, the number of layers, the difference between the temperature of outside and inside surfaces, and the air exposure. For ordinary temperatures and pressures about i cubic meter of air will absorb i calorie in being warmed i degree C., and hence can be considered the equivalent of 1 kilogram of water. The number of calories required for ventilation can then be found by multiplying the number of cubic meters of air by the difference between the inside and outside temperature, and this product by the number of times the air is changed in an hour.

Degree of Warmth.— The temperature inost suitable for healthy persons ranges from 17° to 20° C. for living rooms, and 150 to 18° C. for bed-rooms. For children and aged persons a somewhat higher temperature is required. No standard temperature can be named, because a temperature just comfortable for one person may be too warm or too cold for others. Custom and occupation have a great influence in deciding the matter.

Heat Supplied by Radiating Surfaces.— The heat used in warming is obtained either by directly placing a heated surface in the apartment, in which case the heat

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is said to be obtained by direct radiation, or else by heating the air which is to be used for ventilating purposes while passing to the room, in which case the heating is said to be by indirect radiation (Fig. 22).

Direct heating is performed by locating the heated surface directly in the room, and this surface may be heated directly by fire, as is the case with stoves and fireplaces, or it may receive its heat from steam or hot water warmed in some other portion of the premises and conveyed in pipes. The general principles of heating are the same in each case, but in the case of stoves the temperature is greatly in excess of that derived from steam or hot-water radiators. The heat is carried away from the heated surface partly by radiation, in which case the heat passes in straight lines in all directions and is absorbed by the bodies of persons, by the furniture and walls of the room, without warming the intervening air directly. The heat is also carried away by particles of air coming in contact with the heated surface—that is, by convection—which may be the radiating surface, the bodies of persons, or the furniture and walls of the room which have been warmed by the radiant heat.

The sensations produced by radiant and convected heat are quite different. Radiant heat has the effect of intensely heating on the side toward the source of heat, and producing no warming effect whatever on the opposite side. The heat which has passed off by convection is first utilized in warming the air, and the sensation produced is that of heat equably distributed. Radiant heat and convected heat are essentially of the same nature; in the one case it is derived directly from the source of heat, and at a high temperature; in the other case it is received from the air, which is at a comparatively low temperature.

Resistance to Radiation.—The heat in passing through any metallic substance raises its temperature to an extent which depends upon the facility with which heat is conducted by the body and discharged from the other surface. It is noted that heat ineets with three distinct classes of resistance in passing through a metallic substance: First, that due to the inner surface; second, that due to the thickness of the material; and third, that due to the outer surface. The first and third resistances are due to change in media, and, when the material under consideration is a good conductor, constitute the principal portion of the resistance to the passage of the heat.

Heat Emitted by Radiation.--Heat emitted by radiation, per unit of surface and unit of time, is independent of the form and extent of the heated body, provided there are no re-entrant surfaces which intercept rays of radiant heat. The amount of heat projected from a surface of such form as to radiate heat equally in all directions depends only on the nature of the surface, the excess of its temperature over that of the surrounding air, and the absolute value of its temperature. The rate of cooling due to radiation is the same for all bodies, but its absolute value varies with the nature of the surface. The construction of the ordinary form of radiator is such as to present very little free radiating surface, as all the heat which radiates from one tube to another is reflected or reabsorbed, and is consequently not used in heating the apartment. The greater portion of the heat removed is, no doubt, absorbed by the air which comes in contact with the surface, or by convection. The heat removed by convection is independent of the nature of the surface of the heated body and the surrounding absolute temperature. It depends on the velocity of the moving air, and is thought to vary with the square root of the velocity. It also depends on the form and dimensions of the heated body, and on the excess of its temperature over that of the surrounding air.

Systems of Heating. There are three systems of heating in common use:

(1) Direct, where the heating surface is in the room, as a stove, steam coil, or open fireplace. The heat rays from an open fireplace are radiant. They do not warm the air directly. Heat from a inoderately hot stove or from a steam coil is very little radiant. The particles of air are heated and brought into circulation-heating by convection. This is the cheapest, though probably the least satisfactory, method of heating.

(2) Indirect, where the heating surface is not in the room heated, but in some other portion of the premises, such as a furnace in the cellar. It is impossible to heat by the indirect method without bringing more or less air into the room. It necessitates some ventilation. It is always more costly than the direct method, but is usually more satisfactory and less troublesome.

(3) Direct-indirect, where the heating surface is in the room, but has air coming from the outside at the same time (usually so arranged that the supply of outside air. can be cut off, converting it into direct heating). Directindirect heating is, theoretically, a desirable method of heating and ventilation because it permits the introduction of large quantities of fresh air. The method is objectionable for two reasons—it is responsible for the introduction of considerable quantities of dust, and it is expensive because of the large amounts of air that are brought in through the radiator.

Direct Heating.-Open Fireplaces.—With open fireplaces the heating is almost entirely by radiation, as there is very little opportunity for convection of heat. Its advantages are limited, though important. It adds little of the impurities of combustion to the air of a room, and it ensures the extraction of considerable amounts of the room air. The objections to this mode of heating are that a very large proportion of the heat is lost, and the portion utilized is only given off as radiant heat, thus warming only one side of the body, while the opposite side remains cold. It is also productive of cold draughts, because the cold outside air always tends to flow directly toward the fireplace. The heating is inconstant on account of changes in the direction of the wind. This method of heating is of greatest importance as an adjunct to other systems, such as the heating of the wards of a hospital where an open fireplace is quite cheerful, when the system of heating is by means of hot air or by steam.

Stoves.—The principal advantages of stoves are that a considerable amount of the heat generated is utilized, and the heating is under more direct control and supervision. The disadvantages of this method of heating are that there is a tendency for the air to become dry, and there is no ready means for introducing fresh air. It is objectionable because of the large amount of dust which is produced.

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