It looks like Air-To-Water heat pumps are begining to seep into the American market.
I found one called
Daikin Altherma that extracts heat from air and delivers that heat to the home by way of water, suitable for circulating in a radiant floor. Of course, the lower the feed temperature of the water, the higher the overall COP... So if the radiant floor is optimized for low-temperature feed water, COP will be highest.
The smallest unit is 3 Ton. It has a maximum COP of 4.34. This could be a real game changer.
They even have an optional solar assist kit to go with.
Now, if we can just figure out how to hack those nice, cheap little mini-splits to work with water...
Regards,
-AC_Hacker