In the U.S., 62% of all housing units in the 50 largest metropolitan areas are served by gas-fired water heaters (67% if Florida is excluded), and for the ~4 million gas-fired storage water heaters sold each year to serve these homes only 5% sold are at or above the EnergyStar ® level of 0.67 Uniform Energy Factor (UEF). This is even more pronounced in California where natural gas fuels water heating in approximately 75% of homes, an installed base of 9 million units, one in four gas-fired water heaters in the U.S., and 95% of homes are served by minimum allowable efficiency products of 0.62 UEF. The availability of higher efficiency options is not the issue, with products available up to 0.88 UEF for storage products and 0.97 UEF for tankless products. Instead, this reflects a historic challenge to broadly deploy high-efficiency water heating products, where the combination of lower cost natural gas and higher equipment and installation costs have limited uptake, leaving a large potential for energy efficiency and emissions reductions.

To address this potential, the authors describe an integrated gas-fired heat pump water heater (GHPWH) for residential applications, designed for a direct retrofit and to reduce energy consumption and emissions by 50% or greater over conventional baseline. The GHPWH is based on a direct-fired single-effect vapor absorption cycle using the ammonia-water working pair and incorporates internal heat recovery to achieve a projected 1.2-1.3 UEF, depending on usage patterns. While the design and development of this GHPWH is described in prior publications, here the authors provide an overview of results from an expanded demonstration in Southern California, including as-installed efficiencies versus measured baseline, installation and operating challenges, and qualitative input from installers and the host sites. Additionally, the authors report on a simulation study to quantify the impact of the GHPWH on the home’s HVAC system when installed in a semi-conditioned or conditioned space.