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Hot Water Suply
26. In computing the additional rated capacity for a steam-heating boiler when domestic hot water is to be heated, an approved allowance is 4 square feet of equivalent direct steam radiation for each gallon of domestic water to be heated through a temperature range of 100 degrees; say from 40° to 140° F. Thus, if the house-heating boiler supplies a hotwater heater having a capacity of, say, 150 gallons per hour, heated 100 degrees, the rated capacity of the boiler should be increased by 150x4 = 600 square feet. Where steam circulation and return of condensate are accomplished without a pump, the transfer heater must be as high as possible above the water line of the boiler. Generally, a minimum of 24 inches should be provided between the bottom of the transfer manifold and the water line. Otherwise the condensation may at times be so rapid as to flood the manifold, reducing the heat-transfer rate and lowering dangerously the water level in the boiler.
Electric Water Heaters
27. Heaters in which water is heated by an electric current offer any advantages, as they are clean, safe, odorless and dependable. Since here is no fire or smoke, electric water heaters may be used in places where chimneys are impracticable or where freedom from fire risk is desired.
One type of electric heater is shown in Fig. 13. The water to be heated is contained in the tank a and the heating elements b, which are heated by an electric current, are enclosed in two belts c and d around the outside of the water tank. The tank and elements are heavily insulated as shown at e. The small porcelain unit elements b are clamped in a special steel channel in each belt, which permits replacement of elements without the necessity of draining the tank or interrupting the service. An automatic bellows-type thermostat f controls a switch in the electric circuit and is adjustable for any desired discharge-water temperature between 125° and 165° F.
