This paper proposes a method for creating an extreme seven-day weather data set for HVAC design purposes and examines its suitability. The method involves reviewing data from 20 years of weather observations and selecting a seven-day period during which the weather conditions were extreme. The dry-bulb temperature, the humidity ratio, the solar radiation and other weather elements were obtained for one-hour intervals during the selected period as the design weather conditions. In order to select a meteorologically extreme period with distinct weather characteristics, two weather indices were used. The extremity of the design weather data was evaluated by determining both the frequency of occurrence of characteristic weather values and the likelihood of the thermal load exceeding the design peak load, which was calculated using the design weather data. The results confirmed the validity of using the data to determine sufficient capacities for HVAC equipment.