This study treats the determination of undisturbed ground temperature in a borehole for ground heating/cooling and its effect on the accuracy of a thermal response test analysis. Three different ways of estimating temperatures were used in one groundwater-filled borehole in crystalline rock. The first method, temperature logging along the borehole, is assumed to give the correct temperature profile and results in the best estimate of the mean temperature of the ground. A good estimate is also obtained by circulating a heat carrier through the borehole heat exchanger pipes while measuring the flow temperature at a short time interval (10 seconds). The calculated temperature profile is used for deriving a mean temperature of the borehole. Heat is added to the fluid by friction heat caused by the pump work, which results in an overestimation of the borehole temperature. This influence becomes significant after 20 minutes of pumping.

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