Hydronic Heating Systems:Two-Pipe, Direct-Return System.

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,

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.

Two-Pipe, Reverse-Return System

A two-pipe, reverse-return system (Figure 7-7) achieves a balance in the water supply by creating circuits to the radiators of approximately equal length. Instead of allowing the return supply of water to proceed directly to the boiler from each radiator, the return main carries the water in the opposite direction for a predetermined distance before turning back to the boiler. The first radiator has the shortest supply main but the longest return main. For the farthest radiator, the reverse is true. Regardless of the position of a radiator in the system, the total length of pipe within the circuit of which it forms a part will be essentially the same as that of any other circuit.