Adding to (2), I've worked with liquid-liquid heat exchangers (large heat/chiller units for chemical reactors) and they are super quiet and efficient, but boy h

Adding to (2), I've worked with liquid-liquid heat exchangers (large heat/chiller units for chemical reactors) and they are super quiet and efficient, but boy howdy, are they messy and high maintenance. The glycol lines leak, the water supply leaks, the water needs to be screened and the screen needs to be changed. Air-sourced exchanger individual appliances are just so much simpler for the consumer. Not dissing ground-source heat pumps. Those are fine, since they are typically a plumb-once-and-done. You just generally don't want separate appliances which quick-connect.