The design of a borehole heat exchanger (BHE) field coupled to a geothermal heat pump requires a suitable dynamic model able to take into account the intrinsic time varying behaviour of the building heat load profile and ground thermal response. In this paper a new method is proposed for the calculation of the Temperature Penalty, the key parameter of the Ashrae dynamic method originally developed by Kavanaugh and Rafferty. The proposed method is conceived for maintaining the simplicity of the original Ashrae model while enabling a much more accurate design of the BHE field. The validation of the new procedure is made with reference to a comprehensive set of 240 BHE configurations, described in terms of the corresponding temperature response factors (i.e. g-functions). It is demonstrated that the Ashrae temperature penalty values typically underestimate the corresponding true values with an average deviation of more than 40%. On the other hand the proposed method is able to provide temperature penalty values well centered around the benchmark line and with an average deviation of less than 10%; it is finally demonstrated that the present procedure is also much more accurate than other existing models and simpler to apply in engineering design. (C) 2015 Elsevier B.V. All rights reserved.