Accurate calculation of transient subsurface heat transfer is critically important in sizing ground heat exchangers (GHX) in geothermal heat pump (GHP) systems. The size of the GHX is a complicated function of a number of design variables that include the thermal properties of the subsurface, and the dynamics of short- and long-term building heating and cooling loads. So-called hybrid GHPs have received considerable attention in recent years because they have been show to significantly improve the economics and energy use of GHP systems. Hybrid GHP systems couple a supplemental heat extraction or rejection subsystem to a conventional GHP system to handle some portion of the building or the ground loads, and as such, permit the use of a smaller, lower-cost GHX