Primary/ secondary piping is a versatile piping method that's increasingly being used as the backbone of modern multi-load/multi-temperature hydronic systems.
The concept of primary/secondary (P/S) piping has been around since the mid 1950s. Until the last decade, it was used mostly for larger commercial heating and chilled water cooling systems. But renewed interest in radiant floor heating, combined with increasingly sophisticated expectations placed on residential and light commercial systems, prompted designers to look for a piping method more flexible and forgiving than the standard 2-pipe system. It turns out their predecessors had already made most the fundamental discoveries. During the last few years many of these principles have been "redeployed" and successfully integrated with modern controls. The result is an extremely versatile piping method that's increasingly being used as the backbone of modern multi-load/multi-temperature hydronic systems.
There are at least a couple other conceivable ways to isolate the flow dynamics of one circuit from those of another circuit in the same system. One uses a hot water reservoir tank to serve as a common pressure reference point for each of several circuits piped to it. Another isolates each circuit from the others using heat exchangers. Both of these pale in comparison to the simple elegance of the P/S approach, which makes the piping common to both circuits as short as possible. A pair of closely-spaced tees performs the desired task at a fraction of the complexity and cost of other methods (see Fig. 1).