Open-loop groundwater heat pumps (GWHPs) are considered one of the most energy efficient and environmentally friendly air-conditioning systems for temperate zones. A fundamental aspect in GWHP plant design is early evaluation of the thermally affected zone (TAZ) that develops around the injection well. This is particularly important to avoid interference with previously existing groundwater uses (wells) and subsurface underground structures. Numerical modelling is useful for delineating temperature anomalies. We carry out numerical simulations and a sensitivity analysis for the subsurface parameters affecting the TAZ. Using the simulation results we obtain a relative hierarchy of significance for the parameters with respect to the final result and then apply this analysis to an actual site. The results of the analysis indicate that the hydrodynamic parameters correlated with groundwater flow such as the hydraulic conductivity and the gradient are highly important, particularly those relating to the advective heat flow component. (C) 2012 Elsevier Ltd. All rights reserved.