The methodology for assessing the design outdoor temperatures is based on the principle of accepting a low risk that the extreme outdoor temperatures exceed the design values, However, it was found that traditional design procedures often resulted in over-sized mechanical systems. New approaches were developed to define the design weather data for heating and cooling systems. These explored how the duration of extreme temperatures and building thermal inertia affected the selection of design weather data. This paper describes the development of new design outdoor temperatures for sizing HVAC systems in Montréal, Canada, using both the above factors as well as more recent recorded hourly data for a 15-year period (1974–1988).