Much attention is now focused on utilizing ground heat pumps instead of air source heat pumps for heating and cooling buildings. Since the cost of installing these systems is proportional to the size of the system, modeling such systems is important for understanding, designing and optimizing their performance and characteristics. In order to investigate the thermal interaction of these systems, which is likely to be significant when these systems are installed in a dense manner, the current study focuses on the two-dimensional transient conduction of heat in the soil around vertical ground heat exchangers (GHEs). A finite volume numerical solution is applied on FLUENT. The analytical results of the line source theory are then validated by the numerical results. The performance of multiple boreholes or neighboring systems and their possible thermal interaction is discussed. The effect of system parameters such as borehole spacing as well as heat flux from the borehole wall on the transient response of two ground heat exchangers are discussed. Regarding thermal interaction between two boreholes, results show that for a specific heat flux from the borehole wall, a borehole separation distance can be calculated in order for the temperature of the soil to stay below a desired limit. (C) 2012 Elsevier Ltd. All rights reserved.