The use of natural sources of energy such as solar energy or natural cooling can reduce the energy used by the HVAC systems. The analysis of the energy savings due to the economizer systems, which use “free-cooling”, is carried out for Montreal climate, using hourly weather data monitored between 1974 and 1983. The results can be used in the preliminary design stage of the HVAC systems, and show that important cooling energy savings could be obtained by using the ambient energy.