While the number of installed geothermal heat pump systems is rising legislation is not prepared to address the issues concerning thermal impact. The aim of this paper is to model the effects of geothermal heat pump systems installed to shallow geothermal reservoirs in sedimentary formation based on the results of a real system in order to show the magnitude of the thermal affected zone. The system examined is a standalone geothermal heat pump with a production and an injection wellbeing able to produce a maximum of 3.4 m(3)/h groundwater. The thermal impact determined by simulation was 35 m for the standalone system. Two scenarios were considered to define the impact of two neighboring open-loop systems on each other. The results show that if two systems are to be installed on the same reservoir the minimum distance should be 55 m. That indicates that in case of designing systems installed to similar hydrogeological environment should consider the change in the water table and increased thermally affected zone if other groundwater heat pumps are in the area. (C) 2015 Elsevier Ltd. All rights reserved.