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The excessive pumping power of circulation pumps in vertical ground heat exchanger (VGHE) systems reduces the coefficient of performance (COP) of ground-coupled heat pump (GCHP) systems when buildings are in partial load. A twodimensional thermal network model is presented in this paper to deal with heat transfer in variable flow VGHE systems. To consider the thermal interference between boreholes, the capacitances and resistances are corrected for the different locations of boreholes in the borehole field. The control strategy for the flow rate of the working fluid is to modulate circulation pump speed based on the building cooling/heating load ratio. The comparison of the overall performance of a GCHP system is made between constant flow and variable flow systems in their life spans for a commercial building located in Chicago, Illinois. The result shows that the variable flow system saves about 14% of the annual electricity consumption than the constant flow system during their life spans.