In the present study, the performance of a heat pump unit is measured by varying the secondary fluid flow rate and compressor speed. Test results are applied to design the GCHP system, and then the effects of the flow rate and the thermal conductivity on the performance of the system and the size of GLHX are analyzed. For all compressor speed and operating modes, the COP of the heat pump unit with an increment of flow rate is enhanced, while the increasing rate of that decrease. For low flow rate conditions, the unit COP and the system COP according to the flow rate increase due to a rise of heating and cooling capacity and a drop of power consumption. When the flow rate increases under high flow rate conditions, heat pump unit COP is slightly enhanced in the heating and cooling modes. However, the variation of the system COP in the cooling mode is negligible and that in the heating mode decreases even though the flow rate increases. The GLHX length increases with the increase of the flow rate, while it decreases with an increment of the thermal conductivity. As the thermal conductivity decreases, the increasing rate of the GLHX length according to the flow rate rises. A higher secondary fluid flow rate can increase the performance of the heat pump unit, while it makes the length of the GLHX longer. As a result, it is suggested that the flow rate of the heat pump unit have to be checked to optimize the system, and consequently, the system COP and the flow rate should be considered as the important design parameters in order to reduce the installation cost and save energy in the GCHP system. (C) 2011 Elsevier Ltd. All rights reserved.