What describes a system that is "first fed and first returned"?

A system that is described as "first fed and first returned" is known as a two-pipe direct return system. In this type of system, water or steam is fed into heating elements in the order that they are connected to the main supply line, and it is also returned in the same first-in, first-out sequence. This design helps to ensure that the elements closest to the supply receive heated water or steam before those further away, which can enhance efficiency and performance by reducing the time it takes for services receiving heat to start warming up.

This arrangement provides balanced flow, is relatively easy to manage, and minimizes the temperature differences between the lines. It contrasts with other systems, such as a gravity system, which operates based on natural convection, or one-pipe systems, which may not maintain the same level of balance and efficiency in heat distribution.