Optimization of Electric Hot Water Recirculation Systems for Comfort, Energy and Public Health

Hot water recirculation systems (RECIRC) are labeled green and are sometimes mandated in local plumbing codes. Previous work conducted under non-optimized operation schemes demonstrated that these systems actually waste energy and water versus standard (STAND) water heater counterparts. Optimization of RECIRC system operation by minimizing pump operation did improve energy efficiency 6–60%, saving consumers 5–140% annually in associated utility costs. However, STAND systems were still more energy efficient than any of the RECIRC systems. With respect to factors that might influence pathogen growth, reducing RECIRC pump operations increased disinfectant residual by as much as 560% as compared to the baseline RECIRC system; however, STAND systems still had 25–250% more total chlorine residual than any of the RECIRC systems. At 60°C operating temperature, STAND systems have 30–230% more volume at risk for pathogen growth (e.g., volume with temp 37–46°C) than any of the RECIRC systems. Thus, in the context of “green” design, RECIRC systems provide a convenience to consumers in the form of nearly instant hot water, at a cost of higher capital, operating and overall energy costs. RECIRC systems have distinct advantages in controlling pathogens via thermal disinfection but disadvantages in control via secondary disinfection residual.