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American Society of Heating and Ventilating Engineers Guide, 1936 or tied in with the oil or gas burner switch which starts and stops the burner. For large capacities these units may be installed in multiple. For exceptionally large installations such as central heating plants, cir culating pumps of the centrifugal single stage type, having an average operating efficiency of 70 per cent against heads up to 125 ft, are some times used. It is generally advisable to install the pumps in duplicate to provide for contingencies and to insure continuous operation. In such cases each pump should be made equal to the maximum capacity required. EXPANSION TANKS When water at ordinary temperatures is heated or cooled, its volume is increased or decreased. This variation in the volume of the water in a heating system is generally provided for by means of an expansion tank into which the water can flow from the system during the heating-up periods and from which it can flow back into the system during the cooling-down periods. Fig. 9. An Open Expansion Tank Fig. 10. A Closed Expansion Tank The expansion tank may be open or closed. In an open expansion tank (Fig. 9), the water is subjected to atinospheric pressure and can expand freely without a material increase in pressure. In a closed expansion tank (Fig. 10), the water is subjected to, the pressure of the compressed air within the tank, and as the water expands, the volume of the air in the tank is decreased and its pressure increased. The open expansion tank must be placed at a sufficient elevation above the highest radiator to prevent boiling when the water in that radiator is at the highest temperature to which it is to be heated. For example, if the water is to be heated to 225 F on extremely cold days, the absolute pressure on the water in the highest radiator must be at least 19 lb per square inch. This pressure will be secured if the open expansion tank is located 15 ft above the highest radiator. If a closed expansion tank is used and is located 30 ft below the highest radiator, an absolute pressure of about 32 lb per square inch must be maintained in the expansion tank if the water in the highest radiator is to be heated to 225 F without danger of boiling. 602 Chapter 33--Hot Water Heating Systems and Piping The type of expansion tank used in a heating system, whether open or closed, has no influence on the operation of the system. The only function performed by the expansion tank is to provide for the variation in the volume of the water in the system, and at the same time to maintain a sufficient pressure in the system to prevent boiling when the water is at the highest temperature for which the system is designed. The use of an expansion tank may be dispensed with when the heating system is allowed to float on the water system, i.e., when the connection between the heating system and the water system is kept open so that the water system replaces the: expansion tank. The capacity of the expansion tank should be at least twice the in crease in volume produced when the water in the system is heated from its normal to its maximum temperature. When 25 gal of water are heated from 40 F to 200 F, the volume of water increases to 26 gal. A safe rule, therefore, is to make the water capacity of the expansion tank equal to 10 per cent of the capacity of the heating system. In a forced circulation system, the expansion tank should be connected to the return main near the circulating pump. In a gravity circulation system, the expansion tank should be connected to the flow riser so that air liberated from the water in'the.boiler may escape through the ex pansion tank, except where it is desired to maintain a temperature higher than 212 F, in which, case the connection should be in the return main to prevent possible boiling in the expansion tank. The expansion tank should be protected so that the water in the tank or in the connecting pipe lines cannot freeze. If such water should freeze and the water in the system be heated to cause further expansion, the resulting force will burst the boiler or some other portion of the system. INSTALLATION DETAILS The detailed installation of the pipe system should be governed by four fundamental rules: 1. AU piping must be pitched either up or down so that all gases which are liberated from the water can move freely to a vented section of the system. Whenever practicable, the pipe line should be pitched so that gases flowing to a vent will flow in the same direc tion as the water. When a pipe system cannot be installed without creating air pockets, that is, sections in the system from, which liberated gases cannot escape, such sections must be provided with automatic air relief valves or with air valves which may be operated manually when necessary, or trapped into a pressure tank. 2. .All piping must be arranged so that the entire system can be drained, either to permit alterations or repairs, or to prevent freezing if the system is not to be operated during a cold period. It is well to install a gate valve and union in every riser near the main to permit the draining of individual risers without draining the entire system. It is also well, in large installations, to divide the system into branches and to provide each branch with unions and valves so that any one branch can be drained without disturbing the remaining ones. _________ fo .......uidnuira ui guiles anu providing eacn zone with individual valves has the further advantage of permitting a varying temperature control. For example, if a building is equipped with a forced circulating system and if the south rooms are on one branch of the main and the north rooms are on a separate branch, the valves may be set so that the water will circulate through the north branch with a temperature drop of, say, 10 F, and through the south branch with a tempera ture drop of, say, 20 F, thus delivering less heat to the south rooms than to-the north 603