This kind of water heater is used to heat water under pressure, and to a higher temperature than 130 degrees. It takes the place of a waterback for heating water to be stored in a tank, or may be used as an auxiliary to a waterback to keep a supply of hot water in a tank at times when there is a greater demand than a waterback can supply. Also, it may be used in connection with a steam coil in a tank to heat the water during warm weather or at other times when steam is not available.

In the illustration, Fig. 129, the gas water-heater is connected to a tank. The outer casing is raised to show the construction of the heater which consists of a hollow casting through which water circulates, absorbing heat from the gases of combustion that come in contact with the outside of the casting. Heat is applied at the bottom through a burner that is provided with a mixing chamber to admit air to the gas and thus produce a blue flame.

Fig. 129