If to supply the necessary heat requires the consumption of 20 pounds of fuel an hour and the boiler or furnace is to keep this up for eight hours without attention, it is evident that the fire pot must be large enough to hold 160 pounds of fuel, and in addition the quantity necessary to rekindle a fresh charge, this quantity being ordinarily assumed to be 20 per cent of the quantity of fuel in the fire pot after the firing, or in this case 40 pounds. Hence the fire pot must hold 200 pounds of fuel and still leave space for combustion.

If anthracite coal, bituminous coal, and coke are available and each has such heating value that 20 pounds of it will be required an hour to supply heat, the fire-pot space occupied by an eight-hour charge of each fuel may be figured by dividing 200 by the weight a cubic foot of each fuel, the space thus calculated being approximately 3.6 cubic feet for anthracite, 4.0 cubic feet for bituminous coal, and 5.7 cubic feet for coke. Therefore, if the fire pot were designed for anthracite, it would hold coke enough for a firing period of approximately five hours instead of eight.

Not only is the capacity of the fire pot important, but its depth should receive consideration. If the full-rated load is to be carried without attention to the fire for a minimum period of eight hours, the depth of the fuel-bed should be at least 12 inches. A heater that is to burn coke should be designed for a greater depth-probably 24 inches-on account of the bulkiness of the fuel and the different combustion conditions required for burning it satisfactorily. In fact, one of the largest manufacturers of boilers for heating houses by steam or hot water now designs such equipment for a fuel-bed 18 inches deep when anthracite is to be used.

Regarding the necessary size of fire pot, another detail that affects both economy and convenience of operation is the combustion space above the fuel-bed. Any unburned combustible gases that leave the fire pot are rapidly cooled in passing over the heat-absorbing surfaces between the fire pot and the smoke pipe, and their temperature is quickly brought below that necessary for ignition. If either anthracite coal or coke is to be burned, a relatively small combustion space above the fuel-bed will be required because combustion takes place in or close to the fuel-bed. If bituminous coal is to be used, however, more space should be provided for burning the combustible gases rising from the fuel-bed, or a considerable part of these gases will escape unburned, the flue surfaces will become coated with soot,"and the heat losses will be large.

Another important detail, if bituminous coal is to be used, is the cross section for the gas passages between the fire pot and the smoke pipe. If the passages are not large enough, the draft may be cut down by the accumulation of soot in the flues, possibly to such an extent that the fire will go out. Also, if bituminous coal is to be used, the flues should be of such size and so arranged as to invite frequent and easy cleaning.

Obviously, if the heater is smaller than the proper size, fire at shorter intervals will be necessary, drafts will have to be kept open, and the temperature of the escaping gases will be higher than if a heater of proper size is installed-between 375° and 475° F. in ordinary winter weather., The inconvenience resulting from too large a boiler or furnace is less than from one under size, but careful handling will be necessary to control the fire satisfactorily in mild weather.

Each heater has a particular capacity at which its efficiency is highest, but if the heater be properly designed, within a certain range of capacity the efficiency will be practically equal to the maximum. The equipment selected should be one that during most of the heating season will be operated within these limits, but will take care of maximum demands for a short time.