Multifamily Central Heat Pump Water Heater Field Study

Water heating accounts for 32 percent of energy use in California multifamily buildings and 7 percent of the state’s greenhouse gas emissions. Central heat pump water heaters (CHPWHs) offer an efficient electric alternative to gas, but design complexity, installation, performance, and cost slow their adoption. Single-pass carbon dioxide (CO₂) heat pumps add a specific design challenge: they run best with cold inlet water, while recirculation loops return warm water to the system. This study evaluated how small modular, single-pass split CO₂ CHPWHs perform in multifamily retrofits, and which recirculation configurations work best. The project team monitored five California multifamily buildings with 8 to 50 dwelling units, recruited through the Energy Smart Home program. The sites covered four configurations: no recirculation, return to primary (RtP), RtP with backup electrical resistance (ER) heater, and return to swing (RtS). The team measured hot water delivery, energy use, and system efficiency, and interviewed owners, operators, and the installer. All sites delivered 99.8 percent of hot water draw volume at or above 110°F. Average system coefficient of performance (COP) ranged from 1.9 to 3.2. For those RtP sites with ER backup, the ER heater barely ran, while its standby losses instead lowered system efficiency. Owners and operators reported high satisfaction and no ongoing operational issues. These results show that RtP performs as well as RtS when designers control the recirculation return temperature. RtP with ER backup works as a transitional design toward true RtP, which would need less floor space, electrical capacity, and complexity. Experienced teams installed most sites, so the results of this study reflect best-case retrofits. Future work should define when RtP or RtS uses less energy and demonstrate true RtP in larger buildings.