Which system is characterized as "first fed last returned"?

The system characterized as "first fed last returned" is the two-pipe reverse return system. In this configuration, the flow of water is designed so that as soon as a part of the system is supplied with the hot water or steam, the return is routed through a path that minimizes variations in the flow, ensuring that the last unit to be supplied with heated fluid is also the first to return to the boiler or heating source.

This design helps in maintaining balance within the system, as it ensures that all parts of the system receive equal and adequate heating without the need for additional circulatory devices.

In contrast, the two-pipe direct return system does not follow this same pattern, as it returns the heated fluid through the shortest possible route without specific regard for balance. The monoflow system utilizes a unique approach by allowing multiple heating units to benefit from a single supply and return pipe, relying on special diverter tees to direct flow, which does not fit the "first fed last returned" model. Meanwhile, the gravity system operates based on the principle of gravitational force to circulate water, which does not align with the pressurized methods implied by the other types stated.