It is incredibly difficult to tell which refrigerant Rheem uses for its heat pump water heaters. I contacted the company for clarification but didn’t get a response before publication. Some Rheem documentation suggests certain models use R-410a, despite this being phased out. Other documents suggest the use of R134a. A.O. Smith uses R134a refrigerant for its heat pump water heaters. This isn’t as good as CO2 but is better than R-410a. I expect to see more heat pump water heater manufacturers using carbon dioxide as a refrigerant in years to come. So far, only SANCO models, sold by Eco2 Systems, are available for residential use. Mitsubishi launched a CO2 heat pump water heater in the U.S. in 2021, but this is only for commercial and industrial applications.