This arrangement may be elaborated to almost any extent and is an improvement over the single-pipe system. It is quite commonly used as a method of steam distribution, but it lacks the required elements necessary to a positive circulation. As an example: Suppose that the plant shown in Fig. 6 is working and that the radiator on the first floor is hot, but the valves of the radiator on the second floor are closed and it is cold. The steam entering at the valve _A_ of the lower radiator is being condensed as fast as the heat is radiated. The steam will pass on through the valve _B_ into the return pipe and as soon as the return pipe becomes hot it will contain steam at practically the same pressure as that in the supply pipe. This is what takes place in every working steam plant. Now suppose that it is desired to heat the radiator on the floor above. The steam valve _A_ of the upper radiator is opened to admit steam and the return valve is also opened to allow the water to escape. There is steam in both the supply and return pipes of the radiator below at the same pressure, each tending to send steam into the radiator above at opposite ends. This would make a condition exactly the same as a single-pipe system, with a supply pipe at both ends of the radiator and the result would, of course, be the same as in the single-pipe system. There being no place for the water to escape except against the incoming steam, the water will sometimes surge back and forth with the customary noises peculiar to such conditions. It must not be understood that this will always occur, because systems of this kind are in use with fairly good results, but noisy radiators are not at all rare when working under this condition and the cause is from that described. To overcome this difficulty and change the system into one in which there would be a positive circulation from _A_ to _B_, in each radiator, allowing the steam always to enter at the valve _A_ and escape at _B_, the system must be changed to that of _separate returns_. [Illustration: FIG. 7.--Diagram of a separate return steam system.]