The approximated infinite line source (ILS) model is widely used to interpret thermal response tests (TRTs). It assumes a constant heat flux from the source. However, this assumption is violated in real field conditions by the heat exchange between the circulating fluid and the outdoor environment in an above-ground TRT setup. This results in a fluctuating behavior of sequential estimation and estimation error. In this study, we quantitatively examined the effect of disturbance from outdoor environment on This, especially when TRTs are interpreted by the ILS model, using numerical methods. An analytical model that takes disturbance into account was incorporated as the boundary condition of a numerical model. Using typical synthetic weather data of different seasons and 36 cases of measured weather data, numerical TRTs were conducted and interpreted. Some characteristic behavior of interpretation related to weather conditions was explained and changes in error range with testing duration were analyzed to clarify the applicability and limitation of the interpretation using the ILS model. The results showed that at least 60 h of TRT is required to obtain results within the error range of +5% compared with the reference case. Additionally, some practical suggestions regarding conducting and interpreting TRTs are provided. (c) 2015 Elsevier Ltd. All rights reserved.