In heating, there are two categories of hydronic pipe systems: the single-pipe and two-pipe direct return. The single-pipe system conjures up images of those steam radiators seen in old apartment buildings. They are commonly used small residential, commercial and industrial buildings and are based on a gravity circulation system. Two-pipe direct return systems on the other hand, can be both pump and gravity based and are suitable for buildings of all sizes.
The two-pipe direct return hydronic system has its advantages over the single-pipe system in significant ways. The single pipe system has one pipe running from one radiator to the next and then back to the boiler, flowing in circular motion. Being gravity based, the hot water circulation has a tendency to lose heat momentum as it moves further through the circuit. Essentially, the first apartment on the circuit will be toasty warm whereas the last apartment on the way back to the boiler will be freezing cold because of the water temperature drop.
Two-pipe direct return hydronic systems have a pipe circuit for the supply and the return. The advantage it has over the single-pipe system is that the hot water directly routes to each radiator terminal at the same time. The return circuit carries the cooled water that has been circulated from the terminal back to the pump and boiler to get reheated. This type of circuitry can cause an imbalance in differential pressure due to the fact that length of the pipe is shorter between the terminals closest to the pump and longer at the opposite side of the circuit. Therefore, manual balancing valves with a venturi flow meter are required to maintain an even pressured flow. Two-pipe direct return systems take less time to heat up than single-pipe systems and provide an equal distribution throughout the building.