The house I grew up in had a geothermal system (installed when my parents built the house). I was a bit young to know the precise details, but I believe the ide

The house I grew up in had a geothermal system (installed when my parents built the house). I was a bit young to know the precise details, but I believe the idea was that the ground loop merely heated or cooled (depending on the season) to the ground's ambient temperature, around 60ºF. From there, it was further heated using traditional methods. I think that meant a heat pump, as you mention, but it might have been oil. The energy savings in the winter (US-MA) was nice -- heating from 60 is a lot less energy than heating from 30 or below -- but the summer was the real reason I'll absolutely install geothermal in any house I build: basically free a/c. Not having to think about setting the thermostat for a relatively large house down to 72 (or even lower, if you want to freeze your tookus off) is so freeing.