In Europe there are many heating systems based on the borehole heat exchangers (BHE) and using geothermal heat accumulated in rock formations. Switzerland, which is world leader in this term, has almost 6000 space heating systems based on BHE. Despite wide world experiences in use BHE in Poland does not work any installation bases on BHE. The paper is attempting to show that BHE would be able work successfully in Polish geothermal condition. It would be show bases on inlet data from existed deep wells because use of the existed deep wells helps get the price of installation much chipper. In Poland it is one of the first work basis on practical inlet data and gives opportunity to take account as many working parameters. In practical use the most popular are BHE made as Field's heat exchangers. This device is build of two coaxial pipes with different diameters. Cold-water forces in crack between pipes flows to the bottom and take heat from rocks. At the bottom water has the highest temperature and flows to up by central pipe. Central pipe is usually isolated. Using exploitation dates the heat power of BHE has been calculated to be 45W perm of length. The cooling of the BHE surrounding is the most important problem while calculating thermal power and outlet water temperature from BHE. With time, the cool zone changes its place. The temperature field and its change in time are described by Fourier heat conduction equation. The paper will describe three-dimensional established numerical model of Field's BHE made by using f i n i t e e l e m e n t m e t h o d. To solve the Fouriers equation, the boundary conditions are used: second type (heat flux) and third type (convection heat exchange) which have to be change in time because the heat exchange conditions are changing in time. This model takes into consideration: atmospherie conditions (air temperature, wind speed and insolation), technical work conditions (water flux, water inlet temperature, pipes diameter, insolation thickness) and Iocalisation of BHE (local geothermal heat flux density and rocks thermal properties). As outlet data one obtains distribution of water temperature in crack and inside pipe, distribution of rocks temperature in the surrounding of BHE.