Steam Heating Systems

Among the first central heating systems to be developed were steam heating systems. The early systems consisted of a single large pipe leading from the boiler to a number of metal radiators in the space to be heated. These radiators were heat–transfer devices in which the entering steam condensed inside the radiator into liquid water, and while doing so transmitted heat to the room air. The single pipe not only supplied steam to the radiator, but also returned the condensate (condensed steam) to the boiler. Steam flow took place in one direction and condensate flow took place in the reverse direction. Installation of these one–pipe systems was complicated by the necessity to slope the pipes toward the boiler to assure the flow of condensate back to the boiler. A smoothly operating system was required, however, to prevent the steam flowing over the returning condensate picking up the water and slamming it against elbows to produce "hammering" or "knocking." In later developments, the two–pipe system came into use, in which one pipe supplied steam to the radiators, and a second pipe collected the condensate from the radiators and returned it to the boiler.

The simple one–pipe steam system was not successful in regulating the heat output of the radiators to match changing outdoor weather conditions, so that wide fluctuations in room temperatures occurred. This was caused by the inability to throttle the radiator valve and permit varying amounts of steam to enter the radiator. The valve could only be maintained fully open or fully closed; any intermediate opening of the valve prevented the condensate from leaving the radiator, so that the radiator became filled with cool water.

The two–pipe system, with separate supply and return piping, alleviated this problem of lack of modulated heat output. In this system the amount of steam could be controlled by adjusting the radiator valve so that more or less steam entered the radiator. Each radiator was equipped with a steam trap that permitted the condensate, but not the steam, to flow out of the radiator. Other refinements to the basic two–pipe system, such as the addition of a vacuum pump to return the condensate to the boiler, allowed more economical operation as well as more continuous heat input to the radiators. Steam systems are still widely used in large buildings. Steam can be readily carried over long distances, usually in insulated pipes located in tunnels or mains, without undergoing a change in temperature. In this respect steam has advantages over hot water or warm air, since the latter two mediums undergo decreases in temperature as they are transmitted over long distances. In the case of steam, the temperature remains unchanged, but a portion of the steam condenses as it is carried through a long main and must be drained to a return line and returned to the boiler. Steam systems are not as commonly used in small buildings as in years past, primarily because of developments in hot–water and warm–air systems.

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