The estimation of large wind and buoyancy driven air flow rates in large buildings is essential for the calculation of the annual energy use for cooling and heating. In this paper a new method is presented, how to measure these large air flow rates. The design of a stable CO2 emitting source with large rates is shown. This source has been applied to measure the time-dependent air flow rate in a multi-storey office building, where the five storeys are connected together by an atrium. The measurement results using this new source over a period of one day are shown together with the measurements of the velocity of the air at the inlet of the atrium.