It is generally accepted to-day that the most convenient method of securing hot water in the small house is with the instantaneous type of gas-heater, connected with a boiler for storage purposes, but capable of delivering water directly into the pipes without passage through the boiler, when a sudden demand is made upon it. These gas-heaters have a system of Bunsen-burners which heat the water as it passes through a series of copper coils, and generally the water is warmed to a temperature of 100 degrees in one passage. They are automatically controlled, so that when the temperature of the water goes below a certain fixed standard the gas-burner is lighted by a small pilot-light until the proper temperature is reached, when it is shut off again.
Although these heaters are arranged to deliver hot water directly from the coils, yet if they had no boiler to store up the water, much larger heaters would be required than necessary. For storage purposes, then, a 40-gallon boiler is satisfactory for a residence with one bath and one kitchen, and if there are two baths a 50-gallon boiler is needed. The usual location of the boiler and heater is in the cellar.
However, where there is no gas to be used, the coal-heater must be employed - either the tank-heater or the water-back in the kitchen-range. The latter was the usual old-fashioned method of heating the water, and the boiler was located alongside of the kitchen-range. The size of the water-back was proportioned on the basis of 2 square inches of heating surface to each gallon storage capacity in the boiler. The tank-heater is a special coal-burning stove, designed to serve as an iron-warmer and a water-heater, being usually placed in the laundry in the cellar. Another method of securing hot water, which is not recommended, is to place heating coils in the furnace; it obstructs the fire-pot, chills the fire, overheats the water in cold weather and underheats it in warm weather, and does not operate at all during the summer.