By "ground source", you mean geothermal, is that correct?
Is there a temperature range below which geothermal needs an "auxilliary assist" (i.e., electrical heat strips) as well? I thinking of an area like Pennsylvania, where temps can drop down close to 0 at least a few days of winter.
Also - I see plenty of homes with "regular" (non-geothermal) heat pumps installed around north/north central PA. When shopping for a home that has a non-geothermal heat pump installed, at what seasonal temps does it become impractical or non-economical?