HOT-WATER HEATING Heaters. Hot-water heaters—or "boilers," as they are some times miscalled—are so nearly like the cast-iron steam boilers pre viously illustrated, that it is unnecessary further to describe them here.
Some makers use the same patterns for both steam boilers and hot-water heaters, while others use a higher boiler for steam, giving more 'space above the water line.
Practically the same rules should be followed in selecting a hot-water heater as those laid down for steam boilers. Although a hot-water heater is a trifle more efficient than a steam boiler—that is, more of the heat in the coal is transferred to the water, owing to the temperature of the latter being 40 degrees or more lower than in a steam boiler—nevertheless, practically the same size of hot-water heater or steam boiler is required to heat a given space.
It is well to equip the heater with a regulator, of which a number of good ones are manufactured, in order to control the drafts by variations in the tempera ture of the water, the regula tor being set to maintain any desired temperature in the flow pipe.
Capacity of Heaters. Hot-water heater capacities are based, as a rule, on an average water temperature of 160° in the radiators, when placed in moms to be kept at 70° F.
If the closed-tank sys tem is used, the radiator temperatures may be 220° to 230° or more; hence, if any attention is to be given to the manufacturers' heater rating, the radiation must be reduced to the equivalent radiation in heat-emitting capacity of radiators at 160°.
This is very easily computed, since the heat given off by a radiator is proportional to the difference in temperature between the water in the radiator and the air surrounding it.
This, in the first case, is 160° less 70°, or 90°; and in the other case, say, 225° less 70°, or 155°; that is, one foot of radiating surface at 225° will give off 1 of the heat given off at 160°; therefore, a job with 900 square feet, for example, at 225° would be equivalent in heating power to X 900 — 1550 square feet at 160°, and a boiler with the higher rating would be required It is always well to check the boiler rating as explained under "Steam Heating," except that in hot-water heating only 150 heat units are allowed per hour per square foot of radiating surface.
Of the heat given off by the coal, it is safe to as sume that 8000 heat units per pound are transferred to the water in the heater.
Suppose there are 900 square feet of radiation on the job. Add * to cover the loss of heat from pipes; total - 1200 square feet. Assume that in coldest weather 5 pounds of coal are burned per hour on each square foot of grate; that is, 5 X 8000m 40,000 heat units are transferred to the water in the heater.
The heat given off per hour by the radiators and pipes is 1200 X 150 -180,000 heat units. This, di vided by 40,000, the heat utilized per square foot of grate, equals 4* square feet of grate required.
Some judgment is necessary in assuming the rate of combustion; but this varies from about 3 pounds per square foot of grate per hour in small heaters, to 7 or 8 in larger ones, operated by a regular attendant.