95. Heat escapes from buildings in two ways: first, by conduction through the windows, walls, floor, and roof, and second, by ventilation or leakage of warm air. The loss from the latter cause will depend upon the tightness of the windows and doors and upon the thoroughness of the construction of the walls, especially in wooden buildings. If the outer walls are exposed to the wind, the loss of heat by conduction will be increased from 10 to 30 per cent., while, if they are not wind-tight, the loss by escape of air will be increased to an unknown amount.

96. The rate at which heat will be lost through walls and windows has been found, by careful experiment, to be proportional to the amount of the difference in temperature between the inside and the outside air. The rate of loss under ordinary conditions, and in rooms which have only a moderate exposure to wind, is shown in the following table; the loss is given in B. T. U. per hour, per square foot of heating surface for 1 degree rise in temperature:

Table 15. Loss of Heat

The thickness of the window glass matters very little. The double glass referred to means two sheets of glass with an air space between them.

If brick walls be made double, with an air space in the middle, the air space will reduce the loss of heat below that of a solid wall having an equal thickness of brick. The saving will be about .07 or .08 heat unit per square foot.

97. The losses shown in the table will be increased by-circumstances about as follows: where the exposure is northerly, and the winds are strong, 10 per cent.; when the building is heated during the day, and is allowed to cool off partially during the night, the exposure being moderate, 10 per cent.; same, northerly exposure with high winds, 30 per cent.; when the building is heated only a day at a time, and is allowed to freeze for intervals of several days, such as churches and audience rooms, 50 per cent.

The temperature of cellars that are not warmed may be assumed for purposes of calculation to be 32°. Vestibules and corridors which are frequently opened to the outer air, and which are not heated, may be assumed to have a temperature of 20°.

98. In computing the loss of heat from the air within a room, all of the surfaces must be considered, the ceiling and floor as well as the ends and sides. If a room is located over a cold cellar or a cold corridor, the cooling effect of the floor must be considered. If a room is covered with a flat roof and is ceiled, or lathed and plastered on the rafters, the cooling effect of the ceiling may equal or even exceed that of the walls. If there is a space between the roof and the ceiling which is not wind-tight, the temperature of the ceiling should be assumed to be 10°.

In computing the total loss of heat from a building which is to be warmed throughout, the interior walls, floors, and ceilings may be disregarded, and only the outer walls, windows, doors, roof, and first floor should be considered.

99. In many instances the loss of heat from the room will be partially compensated for by the heat which is emitted from gas lights, and by persons occupying the room. The amount of heat from these sources is about as follows:

In lecture halls and large audience rooms, the amount of heat given off by the audience and the lights may equal or exceed that which is lost through the walls and windows. When this occurs, it becomes necessary to lower the temperature of the fresh-air supply. It may even require to be reduced below the desired temperature of the room, during the presence of the audience. Thus, the actual amount of heat required in any certain case may vary from hour to hour, although the atmospheric temperature is stationary.

The heating apparatus, however, must be capable of maintaining the temperature of the rooms at the desired degree without the aid of the lights and when no audience is present.

The total amount of heat to be supplied by the apparatus should, therefore, be taken as fully equal to the loss by ventilation plus the loss by conduction through the walls and windows or other cooling surfaces.