Buoyancy-driven natural ventilation in ventilation shafts is investigated with a small scale physical experiment within a duct and CFD simulations of an office building. For a fixed exhaust opening, smaller shafts lead to higher flow rates in upper floors of a multi-storey building with a shared ventilation shaft. These higher flow rates are caused by increased vertical momentum within the smaller shafts that induce flow through upper floors, an effect referred to as the “ejector effect”. In the small scale duct, a 0.5 m by 0.5 m shaft leads to a slight reverse flow of 0.0029 m3/s through the upper floor. Holding all other parameters constant and reducing the shaft to 0.25 m by 0.5 m leads to a positive flow rate of 0.012 m3/s through the upper floor. In the CFD simulations of a three storey office building, this same pattern is observed. A 3 m by 2 m shaft leads to a flow rate of 0.0168 m3/s through the third floor, while the reduced shaft of 2 m by 2 m leads to a flow rate of 0.766 m3/s through the same floor. This increased airflow rate from the ejector effect can allow natural ventilation to be used in buildings where it may otherwise have been deemed inappropriate. Most airflow network models neglect air momentum and fail to account for the ejector effect. To improve these models, an empirical model is incorporated into the airflow network model CoolVent in a manner easily transferable to most airflow network models.