I am on the steep uphill side of a learning curve regarding geothermal systems in general. I purchased HVAC-Calc and made it through the model fairly easily. Calculated the heat loss for a home we are planning to construct in TN was about 66,000 BTHh (SIP shell, efficient windows, insulated slabs, etc.). I read somewhere that when designing a radiant floor heat system in our area that the hot water source should be sized to provide 10-20 BTUh per square foot of floor. Doing the rough calculations, between the basement, main floor and conditioned attic, I would need 154,000 BTUh at the high end and about 79,000 BTUh on the low end. In doing the heat loss calcs, I bumped the low temp down from 19 degrees to 10 degrees and feel my numbers were conservative. If the purpose of my design is to replace the BTUs lost through the exterior surfaces would I need to provide that much excess capacity?