I'm starting to worry a bit about your geothermal tech. Like Blain said, the water is definitely not in the 20's. If you have a closed loop, it is on the bottom of the pond, so it is at least in the mid 30's or better. I disagree about the efficiency of an open vs closed loop though. In a closed loop you only have to circulate water, but in an open loop you have to lift water. Then there is the filtering problem.

I understand slightly undersizing the system. An undersized system is more efficient and costs less. That is up to the point where you cannot maintain comfort on all but the most extreme days. It sounds like you may be slightly undersized on cooling, but way undersized on heating. Why don't you unplug your heat strips (like I did) and see if the geo system can keep up on its own? Don't turn the thermostat down while you are away.

Hey Moe, I'm trying to think but nuthin's happening!