In larger buildings, where greater distances require longer runs of pipe and more complicated connections, and where the volume of condensed steam is too great to be taken care of in a single pipe, this system does not work satisfactorily. =Two-pipe System.=--Fig. 5 is a diagram of a two-pipe system. Here, each radiator has a _supply pipe_, through which the steam enters, and a _return pipe_ which conducts the water away. The branch pipes from a common supply pipe or riser, carry steam to the various radiators and all of the return pipes empty into a single return pipe that takes the water back to its source. It will be noticed that in this case the _riser_ also connects at the bottom with the return pipe. This connection is made for the purpose of conducting away the condensation that takes place in the connecting pipes. The water will always stand in these pipes, at the same height as the water in the boiler. The supply pipe from the boiler, and the branch pipes connecting the radiators all slope toward the _riser_. The condensation in the connecting pipes does not pass through the radiators as it returns to the boiler. An exception to this general rule is shown in the radiator on the second floor. In this case the supply pipe slopes downward as it approaches the radiator. To prevent carrying water through the radiator, a small pipe under the left-hand valve connects with the return pipe and the water is thus conducted to the main return pipe. [Illustration: FIG. 5.--Diagram showing the arrangement of a two-pipe steam plant.] Fig. 6 is a simple application of the arrangement shown in Fig. 5. The steam may be easily traced from the boiler to the radiators, and back through the return pipes to its source. The pipe marked _R_ is the connection between the main supply pipe and the return pipe that takes away the condensation of the riser. It is connected to the main return pipe below the water line of the boiler and, therefore, does not interfere in any way with the passage of the steam. Each radiator empties its water of condensation into a common return pipe, that finally connects with the boiler below the water line. [Illustration: FIG. 6.--A two-pipe steam heating plant.]