Most GSHP's have performance charts which rate them at x BTU's at -1*C (30*F) for heating and 32*C (90*F) cooling.
A well designed loop (performance and cost) will get down to -1*C by the end of the heating season. To gain 1* or 2* can cost 75% to 100% more in upfront costs, not to mention the additional pumping costs for the life of the loop.
-10*C is not typical, most are only freeze protected to -9*C.
CJ
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