This paper presents an experimental study of the influence of the water circulation pumps frequency at the indoor and outdoor loops in the overall system performance of a ground source heat pump monitored plant, and describes a three steps new methodology to optimize the system energy performance and obtain the values of the optimal frequencies when ON/OFF regulation is employed. The first step consists of several experimental tests of pseudo-random sequence of frequency steps for both, internal and external circulation pumps, carried out during a single day. The second step is the characterization of the quasi-steady state system performance as a function of circulation pumps frequencies. The final step consists of, by means of an analytical expression, the extrapolation of results to any partial load ratio, what allows finding the optimal frequency as a function of the instantaneous thermal load. The advantage of the proposed methodology is that it can be carried out on site and is able to consider the phenomena occurring at the heat pump and the ground source heat exchanger when the flow rates are varied. The methodology can be applied to any installation incorporating variable speed circulation pumps. Results indicate large energy savings potential for this kind of installation. (C) 2013 Elsevier B.V. All rights reserved.