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284 Chapter 14 1945 Guide Dirt pocket Fig. 35. Piping .Connections to Indirect Radiators with Dry Returns Steam Heating Systems and Piping 285 --Where a building is served by-a-vacuum system or a sub-atmospheric system the stacks should be piped in the usual manner and traps of large capacity, preferably of the combination float and thermostatic type, should be used. In the orifice and closed two-pipe systems, traps should be used on the returns so that a pressure above that of the atmosphere may be secured on the heaters. Each stack should have a separate steam and return connection and trap. Wide stacks are more evenly heated if divided and supplied-with two steam connections, one at each end, and a return connection for each steam connection. For stacks of large capacity it is sometimes desirable to run a separate steam main direct from the boiler to the stacks. Fig. 36. Supply and Return Con nections for Heating Units of Central Fan Systems Fig. 37. Typical Connections to Central Fan System Heating Units Exceeding 12 Sections Fig. 38. Typical Piping for Atmospheric and Vacuum Systems with Thermostatic Control (Central Fan System)