(Even tho my pool's frozen over right now, I still check in. Come on Springtime!)

I think an important distinction needs to made as to what the source of the heat energy is for the heat pump. Clearly we're talking about using the outside air here. We bought our house with an air-source heat pump (Carrier) and an attractive pellet stove. We thought, 'how quaint, it'll be nice to look at on winter days'. Now we know it's essential to heating the house. As said, when the source of the heat energy drops toward, and below, freezing, efficiency goes out the window.

That said, heat pumps can be a very efficient way to heat if one uses sub-frostline ground temps as the source.
There's lots of info on the internets about 'geothermal' energy (50 to 55 degrees) to feed a heatpump and it's something I'm very seriously looking at.

So, more to the point, I wonder if the same idea doesn't exist for heating a pool as well?

Chuck