Among the strategies for borehole heat exchanger (BHE) field design, the Ashrae method is a powerful and fast procedure for the prediction of the BHE field length required by the building monthly heat load profile. The method needs the calculation of the Temperature Penalty, a key parameter that takes into account the error introduced by the infinite cylindrical source (ICS) solution of the conduction problem with respect to the "true" g-function solution. Recently the Authors proposed a modification of the original Ashrae (Kavanaugh and Rafferty) method which maintains the simplicity of the original Ashrae model while dramatically improving the estimations of the required BHE field length and geometry without calculating any g-function and with no need of special algorithms or computer codes. The new method (To method) is centred on a scheme where eight heat sources are thermally interfering with the current BHE source: optimum constants and new model validation had been calculated with reference to a set of more than 200 BHE design configurations described by the pertaining temperature response factors (i.e. g-functions). In this paper the validation of the To model is further carried on in order to prove the model robustness while the design input quantities (BHE depth, ground properties, mutual importance of Ashrae three load sequences) are changed to cover most of practical cases in borefield design. In particular a further simple term is introduced (and discussed) in To Fourier number calculation which enables the present model to assure high accuracy also when the BHE depth is significantly different from usual values, say from short to very deep BHEs (60-240 meters). (C) 2015 Elsevier B.V. All rights reserved.