ERD50
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I'm not sure I really understand that assumption (of not needing to heat the cold output). These systems always use indoor air (and don't draw from outside), right? So, in the winter, clearly, you will have to somehow heat the air that then gets used by the water heater heat pump. Or are we assuming that there is actually an added benefit in the Summer in that the system actually provides extra air conditioning. And somehow the winter/Summer situations are a wash?
As JoeWras replied, it's totally dependent on your particular situation. *if* you have to heat that output, it's a hit on the payback. It *might* get offset by cooling season. It just depends on lots of things, your climate, and where the 'cold' is being dumped.
-ERD50