=144. Hot-Water Kitchen Tank.= A force pump is generally used for pumping water into kitchen tanks, except when water from another tank, such as a city reservoir, flows into it. [Illustration: FIG. 71. Instantaneous water heater.] =145. Instantaneous Water Heaters.= The instantaneous water heater (Fig. 71) is a device which heats water on its way to the outlet. It is composed of a heating unit and piping connected to the outlet pipes. In this type of heater, the pipes must always be kept full of water, and some device should be attached (Fig. 72) to the heater which will lower the heat as soon as, or before, the water reaches boiling temperature. This will prevent steam from forming, which might injure the system. [Illustration: FIG. 72. Device for heating water automatically.] =146. Heaters for Tanks.= Hot water is lighter than cold. A pipe from the bottom of the tank leads into the heater, passes thru the heating coils and up into the top of the tank (Figs. 73 and 74). Water from the tank circulates thru this pipe as the hot water rises and the cold water falls in the tank. As the heater is located on a level with the bottom of the tank, cold water seeking this level flows into the pipe and becomes heated (Fig. 76). [Illustration: FIG. 73. Force pump and boiler.] A booster is a device which keeps the water hot up to the faucet (Fig. 75). If there is a pilot on a gas water heater, be sure to use it. The burners should be cared for in the same way as on other heaters using the same fuel. Keep the tank full of water and the water free to circulate thru the pipes. Air-tight tanks may become so hot that steam is formed in large amounts. Tanks which are not connected with city water pipes may be fitted with safety valves which open when the pressure of steam inside the tank reaches a certain point, which is below the danger point. Should the pipes or tank freeze, do not start the fire in the heater, but thaw the pipes with applications of hot water or other means until the water can circulate in them. Electric heaters are usually incased in a waterproof covering and put in the center of the tank. Small electric heaters are in use for heating a glass or other small amount of water. These are called immersion heaters. =147. The Elevated Water Tank.= In rural homes, water is sometimes stored in an elevated tank. This is usually placed in the attic. It is frequently filled by means of a force pump connected with a windmill or gasoline engine. If there is no overflow to this tank, which there should be, it must be watched when being filled to prevent it from overflowing. It may be fitted with an automatic device similar to those used on the expansion tanks of hot-water furnaces or tanks to water closets for regulating the inflow of water. [Illustration: FIG. 74. Water heater and tank.] [Illustration: FIG. 75. Booster for hot water.]