A ground-source heat pump (GSHP) system uses a relatively constant ground temperature to emit heat in summer and to obtain heat during winter for heating and cooling energy. Among diverse GSHP systems, in designing a closed horizontal system, it is important to accurately estimate the thermal performance of the horizontal ground heat exchanger (GHE). Therefore, in this study, we investigated the performance of a horizontal GHE via experiments and numerical analyses. Thermal response tests (TRTs) were conducted to evaluate heat exchange rates by using horizontal slinky-and spiral-coil-type GHEs installed in a 5 m x 1 m x 1m steel box filled with dried Joomunjin sand. Numerical analyses were conducted for verification with experimental results and a parametric study of affecting parameters on horizontal GHE. The results of the TRTs and the numerical analyses were well matched, and coil-type GHEs were found to perform better than the horizontal slinky-type GHEs. Our results show that, among horizontal GHE design parameters, GHE type and soil thermal conductivity are the main factors to determine the heat exchange rate of a GHE, whereas the pipe diameter does not have any effect on the GHE performance.