I believe you are referring to the primary/secondary piping shown in figure 17 on page 16 of the installation manual. This type of piping layout should be used anytime the flow through the zones does not match the recommended�flow through the boiler. Having a short distance, less than 12",� between the two tees that connect the boiler to the zone piping allow water to by-pass either the boiler or the zones. This means the flow in both parts of the system can be controlled for optimum performance.�