Steam for heating (Fig. 173) is obtained from a boiler fitted with coils of pipe. As the steam passes through the radiator it gives off its heat and is condensed into water. This water flows back into the boiler, either through another return pipe or through the same pipe.

Fig. 172.   Heating by Stove.

Fig. 172. - Heating by Stove.

The double-pipe system requires greater length of piping, but the single-pipe system requires larger piping, so as to allow the condensed water to return to the boiler while the steam is ascending the pipe.