If the required performance is higher than 30 kW it is necessary to control the effective heat capacity of the soil by using the so-called Ground Response Test (on request). During this test a carrier liquid circulates through a probe (usually the first operative one), injecting heat in the soil or detracting it. Meanwhile we measure specific parameters. The injected heat must be calculated and determined in advance, based on the tecnical characteristics of the system. By measuring these parameters, we are able to designate the specific temperature, the thermal conductivity of the soil and the resistence of the probe. Basically the test is performed on a single probe and on the entire system, adapting to various states of stress. A reliable dimensioning can only be obtained if we know the geothermal characteristics of the soil. The features of the underground, the type of probes and the thermal injection are essential parameters for the optimal setup of the installation. They have a strong influence on the number of requested probes, their positioning and the geometry of the surface compound. The determination of the soil characteristics allows us to optimize the usage of the geothermal "reservoir". Using simulation software it is possible to set up the system for longterm operations and to simulate the functionality for e.g. 20 years.