IBC Sl Series Installation & Operating Manual page 59

Each boiler will control its own pump, turning it off or on when heat is required. This approach
saves electricity by reducing the pumping power required as load conditions are reduced. One
boiler control is set up as a "Master" boiler, and up to 23 additional boilers can be added to the
system as "Subordinate" boilers by connecting a twisted pair of wires between the boilers. No
additional controls are needed.
Check valves are used in each boiler's piping to prevent reverse flow when the boiler is off.
Secondary loop sensor to Master boiler
To building
Closely spaced tees
From building
Check valve
Figure 45 Multiple boiler piping concept
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3.11.2 System piping