Optimal window opening based on natural ventilation measurements

From the energy point of view, buildings should be as tight as possible. But lack of ventilation will result in high level of indoor pollutants, which is harmful for occupants. Numerous studies find that lack of ventilation could cause symptoms for occupants, which are characterized by World Health Organization as Sick Building Syndrome.
There are lots of real-time ventilation data and rational ventilation standards in the world. However, these kinds of measurements lacks in China, so more data is needed. In China, most public schools have neither air condition nor mechanical ventilation, so the main form of ventilation is natural ventilation. Thus, it is crucial to do research in measuring the natural ventilation rate in order to improve the indoor environment.
Natural ventilation is mainly driven by heat buoyancy and wind pressure. The lowest air change rate happens when the wind speeds and temperature differences are low. If the quantity of fresh air can satisfy the ventilation standard under these circumstances, the natural ventilation can satisfy most conditions.
According to climate data, the days in which wind speed is below 0.5m/s accounts for about 26%. Most of the school buildings aren’t located in open areas. Furthermore, in consideration of the emergency evacuation for the children, the school buildings are built only with 1-3 floors. So, the disturbances of the surrounding buildings contribute to low wind speeds around the school building.
The time period between cooling and heating seasons usually comes with low indoor and outdoor temperature difference, where most classrooms have open windows for ventilation. It is therefore important to find out whether the ventilation rate is sufficient during this period of time.
The measuring method is based on the release of a stable rate of the tracer gas CO2 given off by solid CO2 (dry ice) in an insulated box. In theory, the dry ice will sublime at a constant rate as long as there is sufficient dry ice in the box. Thus, the dry ice sublimation rate should remain constant at any time once the steady state heat transfer condition has been reached.
The results of the measurement can enrich the database of the Chinese current ventilation situation. The early-design-stage predictions of natural ventilation performance in renovated buildings provide reference to the design of natural ventilation systems.