One-pipe systems are the simplest and easiest hydronic systems to understand and install. As the name implies, one-pipe systems have a single pipe to the radiators, which serves as both a steam supply and a condensate return line. As water in the boiler is heated and transformed into steam, it rises and travels upward through the supply pipe to the radiators. The supply/return pipe is located at the base of the radiator to allow for condensate return. As the steam enters the radiator it displaces trapped air. Each radiator is equipped with an air vent to allow release of this air. Air vents close when the steam reaches them ensuring that only the air escapes and the steam stays in the radiator.
As heat is released from the radiator, the steam condenses and water accumulates. Radiators on one-pipe systems are angled down-ward to allow condensate water to flow back to the boiler, and their pipes are usually greater than 2 inches in diameter to allow the water and steam to pass simultaneously. Since the condensate flows opposite to the flow of steam, these systems can sometimes be noisy.