The US EPA and DOE's Energy Star Program has a brief primer on heat pump water heaters.

 

Mitsubishi Electric put together a more detailed presentation on heat pump technology for the New Hampshire PUD.  

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Currently two members of 100% Renewable Truckee have installed the Rheem Electric Residential Hybrid heat pump water heater with great results. They were installed in an attic and a garage where the air is unconditioned but not as cold as the outside air temperature. These heaters are almost drop-in replacements for your existing water heater as long as it resides in an unconditioned space in your home (like attic or garage). They do require 30amp electrical service to be added. So they are not quite drop-in replacements. Please contact us if you would like more specifics about our experience. One downside of the Rheem water heater is that it uses R410a refrigerant which if leaked into the atmosphere is a powerful greenhouse gas.

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Another good option is the Sanden SANCO2 heat pump water heater. This system uses CO2 as its refrigerant. While CO2 is obviously also a greenhouse gas, it is over 1000 times less powerful than the Rheem refrigerant. The only issue with the SANCO2 is that it is a split unit. There is a water tank which is located inside the building, and there is a heat pump which must be located outside the building. These two units are connected by water pipes which must be less than 50 feet in length. This means that for retrofit applications, it is a more complicated replacement than the Rheem unit. For new construction the Sanden SANCO2 would be a great fit. Please note that some measures must be taken to ensure that the water pipes that run exterior to the building don't freeze.

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