The big plus, in my mind, for air to water is not needing the wells, loop field or pond source. So that cost and logistics goes away. The designer needs to know how the air temperature, required supply temperature interface. As the ambient temperature goes down, so does the efficiency, in heating mode. At a point a backup heat strip, or heat source may need to be enabled. The designer and installer needs to be aware of those relationships. Look to idronics 9 and 14 for more number crunching