I know this may be comparing apples to coconuts, but I have a 5 ton WaterFurnace and over 5000 sq ft. The installer said the pond loops need to be 600' per ton, so I got 3000' of loop. If you really have 500' of pond loop, this may be your problem. You may want to make a call to your system manufacturer and see what they recommend.
My last furnace ran on natural gas, so the first thing I noticed about my geo system is that the air coming out of the registers is not nearly as warm, and the system runs a lot more. My installer told me to expect this from any heat-pump, not just a geo system. That's one reason why you want to leave the thermostat alone. I would still suggest that you disconnect your heat strips, set the thermostat, and see if this works for you. Of course, in single-digit weather, you may want to wait a few days to try this.
