Two-Pipe, Direct-Return System
In a two-pipe, direct-return system (Figure 7-5), hot water returns directly to the boiler from each heat-emitting unit. In other words,
Figure 7-5 Two-pipe, direct-return, hot-water heating system. The water from each heat-emitting unit returns directly to the boiler without passing through other units in the system.
Figure 7-6 Heat-emitting unit with feed line connected to the supply main and return line connected to the return main.
The hot-water supply and return mains are separate pipes. The heat — emitting units are connected to the supply and return lines by separate branches (Figure 7-6).
Each heat-emitting unit represents the midpoint in a complete circuit within a two-pipe, direct-return system. The boiler completes the circuit. The farther a heat-emitting unit is located from the boiler, the greater the length of the piping in the circuit. Although this factor causes problems in balancing the water supply among the various circuits, balancing can be achieved in a number of ways, including (1) balancing cocks, (2) proper pipe sizing, or (3) using a reverse-return system.