Twenty years ago, the contemporary approach to prevent interference between simultaneously operating circulators in a hydronic system was primary/secondary piping. This approach was resurrected from its inception in the 1960s for use in the multi-load systems that appeared as the modern hydronics era took root in the 1990s.
Each load circuit was called a “secondary” circuit and was powered by its own circulator. The secondary circuits merged into a common “primary” loop, which also had a dedicated circulator. The junction between each secondary circuit and the primary loop consisted of a pair of closely spaced tees as shown in Figure 1. The magic of closely spaced tees is what prevented flow in any of these circuits from interfering with flow in the others.