Ground source heat pump (GSHP) systems utilize ground energy for space heating and cooling applications with the help of ground heat exchangers (GHX). When GSHP is used for both heating and cooling applications it is very important to determine the optimum length of GHX. In the present research the length of horizontal and vertical GHX is optimized for a 1.5 TR GSHP system assuming different combination of heating and cooling periods. Eight operating parameters of GHX at three levels have been assumed and L-27 orthogonal array has been employed to obtain experimental trials. For both horizontal and vertical GHXs, Taguchi method has been used to obtain independent optimum length for heating and cooling applications and then utility concept has been employed to estimate a single optimum length for a given weightage for heating and cooling periods. Results were obtained for different cornbinations of heating and cooling periods expressed as weighting factor in utility concept, ranging from 20% to 80% for both type of GHXs. Effect of variation in COP of the ground source heat pump system where the GHX is used is also analyzed to determine its influence on the length of GHX. Length of vertical GHX is found to remain constant for all the combinations of heating and cooling periods, however, the length of vertical GHX is about 33% higher for cooling than that for heating. In Contrast, horizontal' GHX showed about 20% variation at lower and upper bounds and 50% variation exactly at the middle bound. (C) 2016 Elsevier Ltd. All rights reserved.