=Expansion Tanks.=--Fig. 36 is a form of expansion tank in common use. It may be used for either the high-or low-pressure system. The body of the tank is made of galvanized iron and is made to stand a considerable amount of pressure. The gage-glass is attached at _B_, and the overflow at _O_. The pipe _E_ connects the tank with the circulating system and _D_ connects with the cold-water supply as a convenience for filling the system with water. The object in placing the stop-cock _D_ near the expansion tank is to avoid overflowing the system in filling. The overflow pipe, as stated above, is most conveniently connected with the sewer, into which the water will run in case of an overflow, but the other methods shown are commonly used. There should be no valve in this pipe nor in the pipe _E_. [Illustration: FIG. 36.--The expansion tank.] [Illustration: FIG. 37.--When the expansion tank of a hot-water heating system must be so located that it is apt to freeze, it must be piped as a radiator.] The expansion tank must be so located that there will be no danger of freezing. Should it be necessary to place the tank in the attic or where freezing is possible, the tank must be so connected as to become a part of the circulating system. Such an arrangement is shown in Fig. 37. The expansion tank is connected with a supply and return pipe as a radiator. This arrangement is sometimes used but it is not desirable. It is wasteful of heat and there is always a possibility of freezing in case the fire in the furnace is extinguished a sufficient time to allow the water to grow cold. Any possibility of danger from excessive pressures in either the low-pressure or the high-pressure system must originate in the expansion tank. It is, therefore, desired to again mention the possible causes of danger. Any closed-tank system is liable to become overheated. The expansive force of water is irresistible and unless some means is taken to prevent excessive pressure some part of the apparatus is apt to burst. _No closed-tank system should be used without a safety valve._ The low-pressure or open-tank system requires no safety appliances. So long as there is open communication between the tank and the boiler the pressure cannot rise but slightly above that of the atmosphere. There is only one cause that will lead to high pressure in such a system. If the pipe connecting the expansion tank is stopped an excessive pressure might generate. There is little or no danger of this happening. In the closed-tank system the expansion tank should be of greater capacity than for the open-tank system. Its size is commonly about one-ninth of the volume of water used. The larger tank is necessary to prevent too rapid rise of pressure as the temperature of the water rises. The air in the tank acts as a cushion against which the pressure of the expanding water is exerted.