The use of ground-source heat pump systems for heating and cooling of buildings is becoming increasingly widespread throughout the world. The most common application of ground-source heating and cooling systems uses vertical boreholes. The primary function of a ground heat exchanger is to efficiently transfer heat between the ground and an above-ground heating and/or cooling load. The heat transfer process in the borehole is characterized by the thermal resistance of the ground heat exchanger. A ground heat exchanger with low thermal resistance is paramount for the good thermal performance of a ground-source system. A vertical borehole with a single U-tube is by far the most common type of ground heat exchanger in practice. This is due to the low cost, ease of installation and small space requirements of the single U-tube ground heat exchangers. In recent years, some new types of single U-tube ground heat exchangers with innovative designs have been proposed with a view to decreasing the borehole thermal resistance and reducing the required amount of drilling and/or improving the system's thermal performance. One such ground heat exchanger has internal, partly helical ridges that have a non-uniform twist rate to enhance heat transfer. In this paper, we present thermal response test measurements of a standard single U-tube and a single U-tube with internal heat transfer enhancing ridges in a 200 m deep borehole. The primary objective of the work is to undertake comparative investigations of the measured thermal performance of the two types of borehole heat exchangers. The tests were performed with two levels of input power. The first level corresponded to a low-to-moderate level of heat-injection rates to the borehole heat exchanger. The second level corresponded to the peak level of heat-injection rates to the borehole heat exchanger. Two flow rates, each corresponding to a specific pumping power level, were applied for each thermal input power level. The results of this study showed a modest increase in the thermal performance of the enhanced U-tube heat exchanger compared to the conventional U-tube heat exchanger. However, the differences between the two were within the experimental uncertainty.