Economical design of Ground Heat Exchangers (GHEs) relies significantly on accurate estimations of ground thermal properties, including undisturbed ground temperature, ground thermal conductivity, and GHE thermal resistance. Typically these are measured in situ by a Thermal Response Test (TRT), where the parameters are back-calculated by fitting a heat transfer model to the temperature response. The accuracy of most models relies on an assumption of constant ground temperature with depth, but in reality there are several mechanisms by which this may be invalid, one being seasonal variation in ground temperature. In this paper a study of Infinite Line Source Model performance under different seasonal conditions using a Finite Element model is presented. The study aims to determine the robustness of interpretation of TRTs conducted at different times of the year and the sensitivity of the effect's key influencing variables. Results suggest that there is little variability in effective thermal conductivity but some variation in GHE thermal resistance. A dimensionless parameter was derived and found to be strongly linearly related to the difference in GHE thermal resistance, leading to generalized estimates of the potential impact on design length that were greater than 10% under relatively extreme conditions and for short GHEs. (C) 2018 Elsevier Ltd. All rights reserved.