The installation cost and performance of ground source heat pump (GSHP) systems can greatly be affected by application parameters of ground heat exchangers (GHEs). These parameters affecting the helical GHE's performance are their spacing, major diameter and pitch length. In large scale GSHP applications, more than one GHE is needed. Therefore determining the distance between GHEs (spacing) becomes as an important issue. In this work, the effect of distance between vertical helical GHEs on the heat transfer rate (HTR) is studied. Performance of helical GHE is determined for different spacing and the proper distance is examined. Furthermore, the influences of the pitch length (L-p) as well as major diameter (D) of GHE on HTR value are numerically studied in COMSOL environment. The available experimental data are used to validate the numerical results. Computational results show that they are in good agreement with the experimental one. The proper distance between GHEs is suggested as 7 m and more. Furthermore the results of the simulations prove that 100% changes in L-p and D can affect the performance of GHE only in order of 10% although the excavation cost increase more than 10%. The results provide useful guidance for optimum design of vertical helical GHE for GSHP systems. (C) 2016 Elsevier B.V. All rights reserved.