Lower Stand-by losses. Since the FLEX-HEAT system energizes only the number of modules needed to meet the heat loss, less heat is dissipated to the boiler room and less heat escapes up the chimney. Modular systems are redundant. If a single boiler installation goes down, it's a no heat situation. On a modular boiler system, if a single boiler shuts down, the remaining modules will provide heat until the problem boiler can be corrected.
Modular systems can be faster to maintain.
Disassembling and reassembling a large boiler to replace a section can take a large amount of time.