Pressure relief valve (shipped with boiler) typical of each boiler.
Tridicator (shipped with boiler) typical of each boiler.
Check valve (typical of each boiler)
Boiler pump into rear connection (typical of each boiler)
Low-loss header / hydraulic separator (shown) or closely-spaced tees 'bridge' to load
piping
Check valve for opt-out circuit.
Opt-out pump into rear side (opt-out boiler(s) only)
Opt-out load piping in reverse-return for balanced flows