Methods to measure airflow rates using tracer gas in single air handling units are well known. In some buildings however, in particular in Singapore, rooms are often ventilated with two or more units and present large recirculation rates. Large recirculation ratio homogenise the concentrations, so concentrations in supply and extract ducts are close to each other. In addition, these spaces often present a large time constant, so much time is needed to reach steady state. The purpose of this paper is to document an adapted methodology to measure not only the airflow rates provided by each unit, but also the inter-units airflow rates and the global ventilation efficiency, The method is illustrated with an example of measurement in an actual building. It is shown that the most accurate method is not the same for small and large recirculation ratios.