The results of numerical and experimental studies devoted to the evaluation of the effect of phase transitions of pore moisture in the soil mass surrounding the borehole heat exchangers (BHE) on the thermal conductivity of the adjacent soil and on the temperature of the coolant circulating through the heat exchanger are presented. A mathematical model is presented that allows one to describe the spatial non-stationary thermal regime of a soil massif with BHEs, taking into account the processes associated with phase transitions of moisture in the pore space of the soil. This mathematical model is based on the method of accounting the latent heat of phase transitions of pore moisture in the ground by the use of such a parameter as the "equivalent" thermal conductivity. The essence of the method is to take into account the heat of phase transitions of pore moisture in the ground by introducing a new "equivalent" thermal conductivity of the soil, consisting of the direct thermal conductivity of the soil and an additive that is responsible for the freezing / thawing of pore moisture. The methods, equipment and results of experimental studies on the «equivalent» thermal conductivity of soil accounting the phase transition of pore moisture during freezing and thawing performed in laboratory on the test bench simulating borehole heat exchangers working conditions are described. The results of the simulation illustrate the need to take into account the phase transitions of the ground moisture in the ground during the design of BHEs. The effect caused by pore moisture condensation during the operation of BHEs and the associated intensification of the processes of heat exchange was experimentally observed.