GSHP (ground source heat pump) systems have become widely used as a result of the recent increasing demand for new and renewable energy polices in Korea. However, reliability issues have been key issues during installation and operation since they were initially designed. This paper introduces a systematic method for verification of the actual operating performance of a water-to-water GSHP system. The main idea is to compare the actual performance with the manufacturer's data, based on the ISO standard, and then to reduce the gap between the two. The manufacturer's performance data include the EWT (entering water temperature), LWT (leaving water temperature), capacity, flow rate, power, and COP (coefficient of performance). The verification technique was tested using a water-to-water GSHP system designed and installed at the KIER site. The verification study showed that actual performance was lower than that specified by the manufacturer's data. Accordingly, the refrigerant was recharged and the compressor and the expansion valve were replaced, resulting in an increase in the heating and cooling COP by 25.26% and 18.24%, respectively. The new verification method allowed the easy identification of the problems affecting the GSHP system and their subsequent correction. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.