In terms of reducing the environmental pollution caused by effluent water from typical condensers and the water dependency of small modular reactors, indirect dry air-cooled condensers (IDACs) are being considered an ultimate heat sink. While the performance of air-cooled heat exchangers has been investigated thoroughly for decades, evaluations of the condenser performance rely primarily on empirical data. Thus, a method for precisely determining the performance of the IDAC under various environmental and thermal-hydraulic conditions has not yet been understood. The objective of this study is to experimentally investigate the critical parameter that initiates the deterioration of the condenser performance by varying the cooling duty and water velocity. The investigation is also extended to a parametric study of the air-cooling conditions using a best-estimate thermal hydraulic analysis code called multi-dimensional analysis of reactor safety (MARS-KS) to suggest a method for designing an IDAC system. Results showed that, for a given cooling duty and water velocity, the condenser exhibited an insufficient performance above a certain cooling water temperature. The temperature was defined as the pressure transition temperature (PTT) that initiates the increase in pressure inside the condenser. The calculation results of MARS-KS were analysed based on the PTT and was used to suggest methods for designing an appropriate IDAC for the cooling duty and environmental conditions of given target site.