All heat pump water heaters require the presence of a refrigerant to transfer heat from the air to the water. Not all refrigerants are created equal, with some carrying much higher global warming potential than others. I’ve written more widely about refrigerants here. For now, though, suffice it to say that almost all heat pump water heaters sold in the U.S. use R134a refrigerant or R-410a, although the latter is being phased out in the U.S. for lower global warming potential alternatives (like R134a). In contrast, CO2 heat pump water heaters are very popular in Japan, with several manufacturers offering these types of units.