Field Demonstration of Atmospheric Storage Central Heat Pump Water Heater

Central heat pump water heater (CHPWH) systems are playing a pivotal role in the decarbonization of multifamily and commercial buildings. These systems can provide electrified, efficient hot water for both new and existing buildings while minimizing grid impact and energy costs with load shifting capabilities. However, additional production options and design solutions are necessary to overcome remaining barriers to widespread market adoption. Current CHPWH products and designs store the hot, potable water directly in ASME-rated pressurized storage tanks. Despite being the standard approach, this have several inherent issues: they are expensive, a poor use of building space due to their shape and necessary plumbing, are less than ideal for maintaining thermal stratification, are size limited by access corridors in existing buildings, cannot accommodate freeze protection chemicals, and can pose challenges in high-rise buildings with high water pressure. Unpressurized, non-potable storage with heat exchange to the potable domestic hot water loop can mitigate all these issues.Atmospheric storage tanks have several advantages over the traditional approach: Unpressurized storage tanks are 50-75% less expensive than pressurized storage, can be assembled onsite with maximum utilization of available space, allow for the use of freeze protection chemicals that prevent catastrophic heat pump failure, and can isolate heat pumps from the high pressures often seen in mid- and high-rise buildings. Additionally, they have reduced legionella concern since large volumes of warm potable water are not being stored. This study is a field demonstration of the first installation of a return-to-primary CHPWH system application with atmospheric storage. It builds on a previous CalNEXT study ET23SWE0059 ‘Central Heat Pump Water Heater Unpressurized Storage Design Optimization’, which computationally modeled design alternatives to optimize the heat exchanger design for the unpressurized storage tank. The results from the study were applied to the finalized design and its performance will be investigated through a real-world application at a multifamily building. System performance, load shifting, and energy impacts will all be assessed and inform recommendations for manufacturers, programs, utilities, and other stakeholders.