Multifamily Domestic Hot Water Recirculation Survey

Recirculation heat losses typically represent a significant portion of the total load in multifamily buildings with central domestic hot water systems. These losses can reduce system efficiency, cause delivery problems, drive inadvisable corrective actions, and impact the performance of central heat pump water heaters. The objectives of this study were to better understand domestic hot water recirculation loop energy losses, identify system characteristics that influence these losses, and inform design tools and program pathways to address recirculation loads, especially in relation to electrified heat pump water heating systems. The project team conducted field monitoring, recirculation loss calculations, subject matter expert interviews, and qualitative system surveys at multifamily sites across California to help answer these questions. Across the sites recirculation ranged from 24.1 to 99.9 watts per apartment with an average of 62.5 watts per apartment which generally support assumptions in existing heat pump water heater sizing tools. Distribution system modeling of the surveyed buildings found that field-measured losses were 1.1 to 2.3 times higher than idealized calculations. The team suggests that this is mainly attributable to insulation imperfections and cold-water crossover - when pressure imbalances drive cold water into the hot distribution loop. Since no clear correlation between building characteristics and recirculation losses could be established, the study demonstrates that evaluating recirculation performance in any given building requires both quantitative and qualitative system assessment. Using these findings, the team makes several suggestions for management of recirculation losses and central heat pump water heater system design. These include program support of distribution system remediation, combining prescriptive pipe sizing and mandatory insulation levels, proper commissioning measures, and design considerations for minimizing crossover impacts on efficiency and performance. Priority future research includes a controlled crossover quantification study, fixture-level crossover durability testing, and field validation of the optimized return-to-primary configuration proposed in this report.