ET24SWE0061 - Multifamily Domestic Hot Water Recirculation Survey

Temperature maintenance losses often account for around 30% of total thermal loads in centralized domestic hot water systems in multifamily buildings (but can be as high as 60%). Correction of these omnipresent recirculation inefficiencies in high-rise multifamily buildings has direct implications on central heat pump hot water system (CHPWH) design, energy usage, and sizing. A full understanding of the recirculation loads, their driving factors, corrective actions, and their relationship to building characteristics would benefit CHPWH market transformation. Currently, CHPWH design and sizing take a conservative approach in the absence of better information. Often they are generously oversized to ensure that both hot water demand and recirculation losses can be met. There are also limited rebate and program offerings in support of reducing hot water load in preparation for CHPWH retrofits. A full understanding of temperature maintenance losses and possible solutions would enable improvements to existing design tools, expand program offerings, reduce oversizing, energy consumption, associated installation costs, and potentially expand market offerings from systems relying on electric resistance swing tanks.

Additionally, there may be an opportunity for updating or creating new efficiency portfolio measures for existing multifamily buildings. These can be standalone measures for existing buildings or provide added value to CHPWH retrofits. Existing measures covering pumping controls and insulation could be updated, market potential can be quantified justifying tailored marketing efforts for existing programs, and new measures could be possible for interventions such as rebalancing or crossover remediation. A field survey of recirculation systems in existing high-rise buildings will inform recommendations towards each of these ends.

This study will gather field data from 15 multifamily buildings to quantify recirculation loads and their driving factors. Recirculation loads will be measured and correlated to as-built conditions such as pipe sizing, layout, balancing, insulation, and thermally isolating pipe supports. The impacts of each factor will be examined so that existing design tools can be adapted to accurately account for recirculation losses or recommend CHPWH configurations based on a simple set of building condition inputs. This data will be paramount to the expansion of design tools and sizing methodology for systems without swing tanks such as return-to-primary and multipass temperature maintenance configurations. These configurations are being tested in PG&E lab tests and could reduce installation costs, footprint, and energy consumption while expanding commercially available products. All of this would increase market transformation towards CHPWH systems and this field study would be a key step in that direction. In addition to CHPWH goals, the bases for existing measures in eTRM will be reviewed in case the survey data yields an opportunity for updates. The opportunities for balancing, insulation, pumping control, mixing valves, and crossover correction will be explored, with possible recommendations for new measure development.

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.