244. In order to estimate the proper size of a furnace for warming any given building, the first proceeding is to compute the probable loss of heat by radiation and conduction from the whole building, in heat units per hour, with the weather at zero, taking into consideration the situation and exposure to wind. The loss by ventilation must be added to the loss by cooling. (See Arts. 95-99.) Where no definite ventilation system is employed, as in ordinary dwellings, the total loss from both causes may be computed by multiplying the estimated loss from cooling, in heat units per hour, by 2.18.

This is based upon the assumption that the hot air enters the room at 120°, and that all of it escapes at a temperature of 65°. Then it is plain that 65/100 of the total heat (above 0° F.) supplied by the hot-air current is lost, and that only 55/120 is expended in maintaining the temperature of the room. The heat lost by ventilation in that case is 65/55, or 118 per cent. of the loss by cooling, and the total loss is thus 218 per cent., or 2.18 times the estimated loss by radiation and conduction.

The next step is to compute the amount of fuel that must be consumed per hour. This may be found by dividing the total estimated loss of heat per hour, by the number of heat units that will be given off by each pound of the fuel in burning. (Arts. 66 and 67.)

The furnace, however, will not transfer all the heat developed by the fuel to the air passing through, but will waste a large portion of it. The coefficient of efficiency of hot-air furnaces ranges from 50 to 60 per cent.; therefore, the estimate of fuel required must be increased 66 to 100 per cent. accordingly.

The rate of combustion per hour in ordinary furnaces averages about 3 pounds of coal per square foot of grate surface. The required grate area in square feet may be found by dividing the proposed fuel consumption in pounds per hour by 3. Thus we arrive at the dimension upon which all others depend-that is, the area of the grate.

The area of the heating surfaces should be from 40 to 50 times that of the grate, as previously explained.

245. When heat is required at a greater distance than 30 feet, it is advisable to use a combination furnace, and employ steam or hot-water radiators at the remote points. Radiators may also be used to good advantage in corridors and vestibules, where outside doors are frequently opened and where it is difficult to retain warm air.

In large jobs requiring great quantities of hot air, it is customary to employ several furnaces set singly or in groups. In many cases it is better practice to use a number of independent furnaces, instead of one large furnace, or a group of furnaces centrally located. Each single furnace may-then be located in a position which will enable it to serve, to the best advantage, the section of the building assigned to it.

The use of independent furnaces involves more labor on the part of the attendant, but in such cases it is better to provide the extra labor than to sacrifice the efficiency of the heating system, and permit it to fail when heat is most needed.