The purpose of an air-conditioning system is to maintain the comfort of the indoor environment. In practice, to achieve a trade-off between comfort and economy, certain indoor environment risk levels need to be considered. Currently, climate design conditions used in air-conditioning design can only reflect the risk level of extreme meteorological conditions. However, the effect of the air-conditioning system on indoor environment cannot be reflected. Herein, typical office building models of three cities are established and the design load is calculated according to the current HVAC design method. The hourly load is simulated using the hourly meteorological data of 30 years. The indoor environment risk level is evaluated by calculating the ratio that the hourly load exceeds the design load. It's found the highest indoor environment risk level among three cities is only 0.097% when the cumulative occurrence frequency of the climate design conditions is 0.57%. The result indicates the reliability of the air-conditioning system is considerably more than that expected in the current design method. Further research on the building thermal process has revealed neglecting the coincidence of climate design conditions is the main reason for the difference. The research provides guidance for the selection of climate design conditions. (C) 2018 Elsevier Ltd. All rights reserved.