Primary/secondary (P/S) piping has established itself as the default piping technique for multi-load hydronic systems. It's ability to "keep the peace" among several simultaneously operating circulators is what makes it the preferred choice over other types of distribution systems.
The best-known form of P/S piping arranges the secondary loads in sequence along a common primary loop as shown in Figure 1. Such a configuration is more specifically called a series primary loop.