Vertical U-tube ground heat exchangers (GHEs) are a key component in geothermal energy utilization systems like ground source heat pumps (GSHPs). This paper presents a new constant heat flux model of its. Different from the existing uniform constant heat flux (UCHF) model, the model proposed in this paper is a cumulative constant heat flux (CCHF) model. The model is developed by reconstructing an experiment-verified three-dimensional unstructured finite-volume model. Applying the model, the borehole resistance with any geometric structure can be found. In addition, it may be used to develop the short time-step temperature response factors (TREs) for simulating the behavior of the GHE on a short-time-step basis (one hour or less). (c) 2011 Elsevier B.V. All rights reserved.