A steam-heating system, with steam having a pressure less than 10 lb. by the gauge, is called a low-pressure system. If the steam is at a higher pressure, the system is called high-pres~ sure system. When the water of condensation flows back to the boiler by gravity alone, the apparatus is known as a gravity-circulating system. When the boiler is run at a high, and the heating system at a low, pressure, the condens-steam must be returned to the boiler by a pump, steam return trap, or injector.

The low-pressure gravity circulating systems considered to be the best are: the two-pipe system with wet returns; the two-pipe system with dry returns; the one-pipe system, in which all mains, branches, and risers are relieved into a return main below the water-line; and the one-pipe circuit system, in which all pipes are run above the water-line. The choice of any system will depend upon special conditions and requirements