Ground source heat pumps use the ground as the heat source/sink through ground heat exchangers (GHEs). Thermal response tests (TRTs) are conducted under a constant heat rate to determine the effective thermal conductivity of the ground and borehole thermal resistance, both of which are necessary for deciding the size of the GHE. Thermal performance tests (TPTs) are conducted under a constant inlet temperature to investigate the actual performance of a GHE with a complex shape. Although the information from the TPT and TRT is important, both tests are rarely conducted together because they require different test apparatus. In this study, we have developed a cost-effective TPT apparatus by adding a general proportional-integral-derivative (PID) controller and a solid-state relay to an existing TRT apparatus and extending TPT to estimate the thermal properties that can be obtained from a TRT. We called this test a thermal performance-response test (TPRT). The developed apparatus has good controllability with a control error within ± 0.1 °C (0.18 °F) compared to a setpoint temperature. Using the developed apparatus, two TPRTs were conducted in a 50-m-long (164 ft) closed-loop vertical GHE installed in a saturated porous formation at inlet temperatures of 30 °C (86 °F) and 40 °C (104 °F). The changes in the estimated values of the thermal properties and in the heat exchange rate due to the subsurface natural convection were analyzed. The change in the estimated borehole resistance was 8.5% and this led to a small difference of 3.7% in the unit heat exchange rate.