Split HPWHs as an Efficient Solution for Multifamily Buildings with In-Unit Water Heaters

Research output: NRELPresentation


A quarter of all housing units in the United States are found in multifamily buildings, but few efficiency innovations have been made to improve electric water heating systems in this building type. Domestic hot water in multifamily buildings can either be provided by a large central water heater or smaller in-unit water heaters. Integrated heat pump water heaters that are installed in single-family homes are often a poor fit for individual multifamily units because the water heaters are often located in small internal closets. This confined space can limit the airflow to the heat pump, and the cold exhaust air can cause discomfort and reduced efficiency. Split CO2 HPWHs separate the tank from the heat pump, which allows for installation location flexibility. Further, CO2 heat pumps can operate under colder ambient conditions, making them a better fit for multifamily units across a wide range of climate regions. As part of a larger project to characterize split HPWH performance, we evaluated the feasibility of installing an additional, parallel tank that could provide hot water to a second housing unit using a single CO2 heat pump. This approach would reduce the first costs of these more expensive units. Based on the high draw volume that we imposed on both tanks, the same heat pump may be able to support more tanks, depending on hot water usage. This paper will describe the split CO2 HPWH technology, how the two-tank system was controlled, and results from our laboratory evaluation.
Original languageAmerican English
Number of pages15
StatePublished - 2022

Publication series

NamePresented at the ACEEE Summer Study on Energy Efficiency in Buildings, 21-26 August 2022, Pacific Grove, California

NREL Publication Number

  • NREL/PR-5500-83834


  • CO2 HPWH
  • heat pump water heaters
  • HPWH
  • multifamily buildings
  • split HPWH


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