The use of renewable energies, and of geothermal energy in particular, is increasingly being applied in Germany and Europe for the development of new residential districts. The use of geothermal borehole heat exchangers (BHE), in combination with ground-source heat pumps (GSHP), represents an important part of shallow geothermal systems, which are used, among other systems, in urban areas due to their small space requirements. Over the course of planning BHE systems, performance must be determined via the parameters of thermal conductivity, thermal capacity, undisturbed ground temperature, and borehole thermal resistance. These can be identified by the experimental approach known as thermal response testing (TRT). The thermal parameters change due to the influences of the seasonal temperature fluctuations that take place in the ground. In this paper, a pilot double-U BHE heat exchanger field with a depth of 120 m was investigated from this perspective. TRT was carried out using monthly measurements taken over the period of one year using an electrically powered mobile TRT device. The evaluation of the individual tests was carried out using the line-source, moving-line-source, and cylinder-source theories. Our results show that the season in which TRT was implemented had an influence on the determined thermal parameters, with better thermal conditions being obtained in winter months. This is especially visible for thermal conductivity, with monthly deviations of 0.1 W/(m∙K), independent of the evaluation approaches used.