In Alaska we have a fairly sophisticated solar heat system. So sophisticated it doesn't work at all; not after its first several months - and that was some time ago. But this thread isn't about Alaska. Our rural Arizona home is set up like many and perhaps most that are not on a municipal water grid. We pump water from the aquifer a zillion feet u/g and store it in an underground cistern; ours is 2,550 gallons (9,600 liters). A second well pump there provides pressurized water for household use but effectively, the cistern always is close to full. So I was contemplating a geothermal system the other day and now am thinking that a supremely efficient system would be one that has its circ'n lines embedded solely inside that cistern. It seems to me that the heat transfer capacity of an u/g reservoir of that size not only would be superior to a more conventional one by orders of magnitude, but the cost of installing it would be staggeringly lower, and its reliability far greater. The reason is that the heat specificity of water or, rather, its ability to transfer heat, is much, much greater than dry or even moist soil. Not only that, but consider the uses of domestic water: most of it is for cleaning: dishes, clothes, bodies. For only a very small amount of usage is cool water desired (that glass of water; the ice maker), so a system like this would provide the additional energy effiiciency of lowering one's water heating cost. I don't have a clue as to what the dT would be, but I'm thinking that something like going from 54ºF to 70-75º is probably in the ballpark. In the wintertime a heat pump runs in the opposite direction, and this set-up under those circumstances would of course be extracting heat from that cistern to warm the house. If my concept works at all it would be one far more suited to AZ, TX, FL and SoCal than for Minnesota....or Alaska. (The way around that conundrum would likely be to use other ways to provide winter heat). So, a Q for all: does anyone know if this is a Known Possibility, and whether it is used anywhere? Are there pitfalls I haven't considered?