One consideration for heat pump water heaters in northern climates -or other heating dominated climates- they take heat from the air inside your house and when your heating system is running that is what is heating your water with the added inefficiencies of the the heat pump.
I have one in a heating dominated climate because the math still worked out as less expensive averaged over the year.
They can be ducted to dump the cold air outside. It requires a 6" vent pipe.
I've thought about doing it, but I doubt I will after feeling the cooling impact. The cooling impact is fairly minor overall. Although maybe I'll come up with an "I'm bored" project and figure out how to vent it in the winter but keep the cool air in the house in the summer.
I have also thought about ducting mine; it would be nice in the summer to put the cold air anywhere but the basement, but in the winter I'd rather it be in the basement. So its a task I have left also for a later time. Ducting it outside will result in an equal amount of air movement from outside to inside, depending on climate and how much air leakage there already is that may or may not be an issue.
But, that wasn't really what I was trying to get at. Here is a scenario, the outdoor high temperature is 32*F and one is heating their house with natural gas to 68*F and you water heater is taking heat from the air inside your house to heat the water to 120*F. During that time the water is heated with natural gas via the heat pump with its additional running costs and distributing the heat to the location of the water heater; it would be most cost effective during that period to heat directly with one's primary heat source.
For me the free heat in the air during the summer and the load taken off the basement dehumidifier mean that it is still cheaper overall to run that one fueled by my boiler. The price difference just isn't as much as if I lived somewhere that the heat almost never ran.
