Light wells in the centers of high-rise apartment buildings in Japan are called `Voids'. Gas water-heaters built into Voids discharge exhaust gas so a large enough opening has to be designed at the bottom of a Void to keep the indoor air quality (IAQ) acceptable. In order to secure the IAQ in the Void from contamination, a simple calculation method of the ventilation rate induced by wind force and thermal buoyancy through openings at the bottom, along with heat sources such as water-heaters, is presented. The accuracy of this calculation method was examined by wind tunnel testing. As a result, it turned out that the simple calculation methods introduced in this study were valid for predicting the vertical temperature distribution and ventilation rates in Voids.