Residential heat pump domestic water heating systems (HPDWH) use synthetic refrigerants with high global warming potential (GWP).  In the 1990s, Japanese engineers developed an alternative HPDWH technology known as “Eco-Cute” that uses CO2 for refrigerant, a naturally occurring gas with global warming potential (GWP) 2,000 times less than synthetic. Oregon’s plumbing code requires hot water heating systems to be tested and listed by an approved agency (such as Underwriters Laboratories UL).  Although it is not yet UL listed, the City of Portland allowed us to install the CO2 HPDHW system as an alternate material through its Alternative Technology Advisory Committee process.