A variable capacity heat pump, with variable speed drives on the secondary circuits, needs an adequate control for achieving the maximum energy efficiency. In the present paper, experimental data and a modeling procedure of a ground source heat pump (GSHP) with variable speed compressor, variable speed water pumps and variable speed fans in the coils are reported. Both data and model are used to evaluate the operating conditions that lead to the maximum seasonal coefficient of performance of the system. The control parameters that can be varied are the followings: frequency of compressor, frequency of water pump to the borehole heat exchangers, frequency of the water pump to the user, velocity of the fans, water temperature to the user. From the experimental data, it was possible to define a control variable which is the ratio of compressor electric power consumption to the power absorbed by the water pumps. The model is also the baseline to develop a control strategy that is implemented in the present GSHP system, developed in the framework of the European Project Ground-Med, with the final objective of maximizing the seasonal coefficient of performance of the system. (C) 2015 Elsevier By. All rights reserved.