=Separate-return System.=--A diagram of a _separate-return_ system is shown in Fig. 7. In this figure, the radiator, boiler and supply pipes are the same as those of Fig. 5, but there is a separate return pipe from each of the radiators, connecting with the main return pipe at a point below the water line of the boiler. Examination of this diagram will show that there is an independent circuit for the steam through each radiator. The steam is taken from a common riser as before but after passing through the radiator the water is returned by a separate pipe to the main return pipe at the bottom of the boiler. Fig. 8 is an application of _separate-return_ system. It is exactly the same as Fig. 6, except that each radiator has an independent return pipe. Steam must always enter the radiators at the valves _A_ and leave at the valves _B_. This makes a positive circulation that renders each radiator independent of the others. There is no opportunity for steam to pass through one radiator and interfere with the return water of another; it, therefore, prevents the possibility of hammering or surging so common in poorly designed steam systems. Of all the methods of steam heating where the water of condensation is returned to the boiler by reason of gravity this is the most satisfactory. This plant requires a larger amount of pipe than the other systems described and as a consequence the cost of installation is greater but it repays in excellence of service the extra expense incurred. [Illustration: FIG. 8.--A separate return heating plant.] =Overhead or Drop System.=--There is yet another gravity system of steam heating that is sometimes used in large buildings where economy in the use of pipe is desired; this is the _overhead_ or _drop_ system shown in Fig. 9. It is not a common method of piping and is given here only because of its occasional use. In the arrangement of the _drop_ system, the supply pipe for the radiators rises from the boiler to the highest point of the system and the branch pipes for the radiators are taken off from the descending pipe. Its action is the same as that of a single-pipe system but the advantage gained by the arrangement is that the steam in the main supply pipes travels in the same direction as the returning water of condensation; the cause of _surging_ in long _risers_ is thus eliminated. The two-pipe systems of steam heating are more certain in action than the single-pipe methods because there is nothing to interfere with the progress of the steam on its way to the radiators. In long branch pipes of the _single-pipe_ system, the returning water is frequently caught by the advancing steam and carried to the end of the pipe, when _slugging_ and _surging_ is the result. [Illustration: FIG. 9.--Diagram of the overhead or drop system steam plant.]