Experimental Study on Inhibition of Legionella Bacteria in Sealed Vessels for Higher Efficiency of DHW Heat Pumps
This study explores the feasibility of lowering the set temperature in domestic hot water heat pumps (DHW HP) systems to promote energy conservation. The current JIS standard requires a set temperature of 65°C to prevent Legionnaires’ disease, based on guidelines from the Ministry of Health, Labor and Welfare. However, these guidelines were originally intended for open storage tanks and circulation heating systems, such as those in public bathhouses, where water is exposed to air and reused. A previous experimental study has reported that reducing the set temperature by 10°C can improve the COP of DHW HP by approximately 10%, contributing to substantial energy savings. In contrast, domestic DHW HP systems use sealed storage tanks and once-through heating. In sealed tanks, water is isolated from air, making it difficult for aerobic Legionella bacteria to enter or proliferate. Furthermore, once-through systems prevent used water from reentering the system. These structural differences significantly reduce infection risk. Experimental results demonstrated that Legionella pneumophila is highly sensitive to combinations of high temperature, pressure, and chlorine. Specifically: (1) Under pressure above 0.17 MPaG and chlorine = 0.1 mg/L, bacteria were inactivated by over three orders of magnitude within 10 minutes; (2) Between 46–55°C and chlorine 0.05–0.4 mg/L, synergistic effects of temperature and chlorine were observed. (3) These effects were further enhanced under high-pressure conditions. Importantly, such temperature reduction strategies can be implemented through software-based control modifications, without increasing equipment cost, while maintaining microbiological safety. This makes the approach both energy-efficient and economically accessible to consumers, which is especially relevant as the DHW HP top-runner standards continue to evolve.