Hot water circulation (HWC) systems in multifamily buildings face a fundamental trade-off: maintaining temperatures sufficient to suppress Legionella pneumophila (>= 50 degrees C) while minimizing the 2.5-4.3 TWh annual energy loss these systems represent in Sweden alone. This study employed a novel dual approach combining controlled laboratory experiments with real-world validation to address this challenge. We constructed a fullscale test rig simulating a 20-apartment building to quantify thermal losses and microbial dynamics under varying flow rates and temperatures. This was complemented by a field validation encompassing 56 water samples from 31 multifamily buildings. The results demonstrate that when optimizing the system to maintain a regulatory required return temperature of 50 degrees C, thermal heat losses were nearly identical between low-flow (0.2 m/s) and high-flow (0.5 m/s) operation. The decisive factor was pump energy, where high-flow operation required 3.4 times more power than low-flow operation (108 W vs. 32 W). This resulted in a total annual energy saving of approximately 12% for the low-flow strategy, entirely attributable to reduced electricity consumption for the pump. Periodic thermal shocks at 60-65 degrees C effectively reduced L. pneumophila concentrations, indicating that continuous high-temperature operation is not required for microbial control. Field sampling revealed that 23% of samples tested positive for legionella, with problematic cases strongly linked to design flaws like towel warmers connected to the HWC loop. These findings indicate that a risk-based strategy combining low-flow circulation (0.2 m/s), a baseline return temperature of 50 degrees C, and periodic thermal shocks can significantly reduce system energy consumption while maintaining legionella safety.