If you've seen any of my other posts, you know that I installed a 5 ton inverter series MrCool geothermal system earlier this year, and have had nothing but problems with it. I just found out that we are getting a natural gas line down our street in the Spring, so I plan to switch to a furnace and standard AC. Other than the problems I have had with the MrCool system, another big driver for me is that the cost per KWH of heat will be about the same, but with the benefit of "instant heat" coming out of the registers with the furnace. Briefly, I calculated that with a 95 AFUE furnace, the "COP" of the furnace versus electrical heating would be 3.48. This is with electricity costing 25 cents per KWH and NG costing $2 per therm where I live. You can check my math, but I think I got it right. My current geothermal system is open loop and, if I include the cost to pump the water from the well, the realized COP is 3.5, which makes the running cost per KWH the same as the furnace. I'm either going to get a single stage furnace and AC, or a two stage system. No more inverter compressors for me. As much as I would love to install the new system myself, I plan to have it installed by a contractor so that it is covered by warranty. I expect my summer air conditioning costs to be comparable, as pumping the well water brings the cost efficiency of the geothermal down to the standard AC level.