Better energy efficiency of ground coupled heat pump systems in comparison with traditional applications leads to continued growth in the number of installations for space conditioning. The solution with vertical heat exchangers is the most widespread. The design of the borehole heat exchangers (BHEs) is a nodal point, both from an energy efficiency and an economic point of view. In literature, several methods to design these systems are available. However, to promote their application, easy-to-use procedures are required; furthermore designers very often are discouraged from the use of not-open or complicated computational tools. Among the literature models, the ASHRAE method is surely the simplest procedure and, as a consequence, it is suitable for this goal. In this approach, sizing of the BHE length is strongly affected by the parameter named penalty temperature, which is an index to evaluate the long-term behaviour of the borehole field. In this paper, a review of this index is reported and in addition a new approach for its evaluation is presented. Furthermore, a detailed analysis of the proposed method is performed on a real case-study building with only heating conditioning. (C) 2012 Elsevier B.V. All rights reserved.