In recent years horizontal Ground Heat Exchangers (GHEs) has attracted a growing interest as heat source/sink for ground source heat pump systems. Horizontal GHEs initial installation costs are lower than the vertical ones; however, they require larger land area and more pipes. Therefore, it is necessary to reduce their required land area and pipe length by improving their thermal performance. In this study, a numerical modal based on 3-D simulation of GHE with computational fluid dynamics methods is developed. The model is used to evaluate the thermal performance and initial installation cost of horizontal GHEs. Four different types of horizontal GHEs: linear, spiral, horizontal and vertical slinky, and different soils types are considered. Obtained results indicated that the spiral and linear configurations have the lowest initial installation costs in single and parallel arrangements, respectively. Furthermore, a new design concept based on applying secondary soil with better thermal properties near the GHE pipes is introduced. It is shown that applying the secondary soil can improve the thermal GHE performance and reduce the initial installation cost of the horizontal GHE, when the thermal conductivity and volumetric heat capacity of the secondary soil is greater than those of the background soil. © 2018 Elsevier Ltd