A ground heat exchanger is the essential part of a ground source heat pump system. The soil temperature profile strongly influences soil properties and ground heat exchanger, which vary with time and space. The soil temperature profile is also a function of the heat transfer rate extracted from/or transferred to soil. Although there are several studies on the sizing and selection of ground heat exchangers, they either use the unchanged soil temperature obtained from meteorological data or do not run for long time periods to obtain a steady periodic soil temperature profile and, thus, over-predict the ground heat exchanger performance. The aim of this study is to obtain transient soil temperature profile of a parallel pipe horizontal ground heat exchanger by considering various system parameters with realistic boundary and operating conditions using meteorological and hourly building data. Experimental results have been obtained from a GSHP system established at Yildiz Technical University and compared with the results of numerical analysis. For a case study, the hourly required heating and cooling loads of a 200 m(2) office in Istanbul all year round are calculated by using HAP software. The fluid inlet temperatures equivalent to the hourly need for heating and cooling loads of office throughout the year have been simulated for a ten-year period in accordance with the different heat amounts extracted from soil or transferred to soil per unit pipe length. The effects of burial depth, pipe spacing and surface effects on soil temperature are also investigated. The horizontal and vertical temperature distribution in soil at the beginning, middle and end of heating and cooling seasons of first, fifth and tenth years are represented. Considering initial and operating costs, a reference function is defined as an optimization parameter. The effects of the increase rate in electricity prices, number of parallel pipes (NPT), burial depth, pipe spacing, pipe diameter and pipe length on reference function are investigated. (C) 2018 Elsevier B.V. All rights reserved.