An energy pile is a novel Ground Heat Exchanger (GHEX), which is an economical alternative to the conventional closed-loop vertical GHEXs. Because energy piles can significantly reduce initial investment costs by being equipped in existing pile foundations, there has been a growing trend to use energy piles as GHEXs. However, prevalent design methods or commercial design programs for GHEXs are inapplicable to energy piles because they cannot take into account the large volume of concrete filling material and the complicated configuration of heat exchange pipe. In this study, a new design algorithm is provided for cast-in-place energy piles with the aid of the PILESIM2 program. When the specifications of energy piles and ground conditions are provided, allowable heating and cooling loads can be determined by the developed design algorithm along with satisfying the design range of Entering Water Temperature (EWT) to achieve the maximum efficiency of a heat pump system. Then, the allowable heating/cooling space area that can be covered by the designed energy piles or the expected portion of energy demands for the considered space can be evaluated step by step. In order to verify the developed design method, two-year heating and cooling operation was carried out in the six cast-in-place energy piles constructed in a test bed. Based on the design output, a Ground Source Heat Pump (GSHP) system was installed for heating and cooling the office space with an area of 103.74 m². During the heating and cooling operation, not only thermal performance of the GSHP system but also temperature variations of the working fluid and the surrounding ground were continuously monitored. In addition, the economic feasibility of cast-in-place energy piles compared to the conventional GHEXs was evaluated.