Borehole heat exchangers (BHE) are made as hydraulic systems consolidated by cement slurry in boreholes. They mediate in heat exchange between the receiver on the surface (heat pump), and the reservoir of thermal energy (rock mass). In the analytical description of temperature changes accompanying energy download and return to the rock mass, the temperature of the entire borehole heat exchanger is assumed to be the same. In fact, the exchanger temperature changes both in the vertical and horizontal cross-sectional view of the exchanger. Accordingly, the temperature of the interface of the hardened cement slurry and the rock mass differs from the one which was adopted for design calculations. The article shows the differences between the theoretical and actual state, when the temperature was measured with a thermal imaging camera. It also includes suggestions regarding the mathematical description of temperature changes on the side surface of the heat exchanger. Measurements were made by a model of a BHE equipped with a single U-tube.