In this system the main supply is carried to the ceiling of the cellar and there makes a complete circuit of the building at a downward pitch and connects again with the boiler below the water line. (Fig. 60.) From this circulation pipe single risers are taken to the radiators, and the condensation flows back through the same pipes to the circulation pipe, and is carried along with the steam and returned to the boiler. This circulation pipe should be of large size and with a good pitch all the way.

Fig. 60. One Pipe Circuit System.

Fig. 60. One-Pipe Circuit System.

In all single-pipe systems the piping must be larger than in the double-pipe systems, as the steam and the returning water are flowing through the pipes in opposite directions at the same time. This may be obviated by running a single riser to the top of the building and then branching out to supply the different radiators by single pipes run back to the basement. In this case the steam and water flow in the same direction and for large work this is often done.