In a typical ground-source heat pump system utilizing a vertical ground heat exchanger, the horizontal piping connecting boreholes can exceed 10% of the total length of the vertical installation. In designing such systems, the effect of the horizontal piping on the thermal performance of the system is typically neglected; however, there has not been an effort to quantify the effect of this assumption. This work studies two buildings in two locations with a simulated vertical ground heat exchanger, both with and without the addition of a horizontal ground heat exchanger, to determine a first-order approximation of the effect that the horizontal piping has on the performance of the system as a whole. The results show that, using a ten-year simulation period, the inclusion of the horizontal piping in the simulation leads to a mitigation of heat pump entering fluid temperatures of up to 0.8°C (1.4°F). Additionally, the horizontal ground heat exchanger is found to be equivalent to up to 30% of the installed vertical length for the cases studied.