This paper describes a small-scale experimental direct-expansion ground source heat pump system in typical clay soils for space heating of a building in Jinzhou, China. It uses R22 as the refrigerant, consisting of four single U-shaped copper ground heat exchangers with a uniform depth of 20 m. A special circular bend is designed on the copper tube to enhance the lubricant oil return of the scroll compressor. The operation performance and energy efficiency of the DX-GSHP system is tested in the heating mode. Results show that the present DX-GSHP system is reliable and effective. During the heating period, when the heating capacity of the indoor fan coil is 6.41 kW, the room temperature varies from 18 to 20 degrees C even if the door is opened or closed frequently. The average COP of the heat pump unit and the whole DX-GSHP system are 3.12 and 2.88, respectively, and the average heat-transfer rate of GHEs per depth is 54.4W/m. In addition, the influence of the opening of the electronic expansion valve on the operation performance is analyzed. The present results can provide useful guides for the design and further improvement of DX-GSHP systems under similar geological conditions. (c) 2013 Elsevier B.V. All rights reserved.