Yeah, efficiency drops off quickly the lower the temps but they have made serious gains in efficiency the last few years. My intent would be to replace the ac units and have it to augment the furnace. I haven't decided whether to get a geo one or not.
A Geo one *will* work in lower temperatures as it draws heat from underground as opposed to air. Much better system to be honest.
Now that I think about it maybe I should replace my natural gas unit with a geothermal. I wonder if it's worth it. Hm......
i have geothermal for heat/cooling and it also has a desuperheater so when the AC is on it dumps extracted heat into a 50 gallon water tank that feeds the hybrid hot water tank. 2000 feet of pex loop in the back yard to sink the heat/cold.
if you can diy it thats like 1/3 the cost probably. you also need really good ducting (as in BIG diameter runs and everything insulated/sealed or youll likely just waste the savings.
love it.