Here are summarized the results of an extended monitoring campaign on three pilot sites in Valle d'Aosta region, located in the northwestern alps, to evaluate energy performance referred to heat extracted and temperature at different depths and with different systems. Experimentation results led to prepare a guide lines aimed for designing properly geothermal plants in a mountainous region, thus focusing on probes spacing, minimum depth of boreholes related to altitude, occurrence/absence of groundwater, morphological factors, practical sizing and indication about operation modes as a function of the hydrogeological conditions.