A primary secondary loop divides a hydronic system into two independent circulation circuits connected by a common pipe, allowing separate pump and flow rate control for each loop. The primary loop serves the boiler while secondary loops serve heating zones or individual buildings. This configuration enables flexible system design, equipment isolation, and simplified balancing.
System Configuration
The primary loop typically maintains constant 15 to 20 GPM through the boiler regardless of secondary demand. Secondary loops operate with variable flow rates between 2 and 15 GPM controlled by zone thermostats and individual zone pumps. Connection points use tees on the main supply and return piping, separated by minimum 12-inch straight pipe sections.
Installation Advantages
Primary secondary loops prevent short-cycling of the boiler by decoupling it from zone demand fluctuations. They eliminate the need for 4-way mixing valves and simplify complex multi-zone installations. This design is standard in commercial buildings and preferred for residential systems exceeding 40,000 BTU per hour heat output.