CO2 Heat Pump
Operates at much higher pressure - 600-1800 PSI. COP is 4. 
- SanCO . Down to -20F. 5x more efficient than electric heat for water heating. Appears workable, ROI in 5 years, . $5600 for a hot water heater. 
- Goes down to -4F 
- There are heat pump water heaters available. This one uses backup resistive (1.5kw) and heat pump (2.2kW) heating. It's a hybrid heater.