Air leakage or controlled ventilation?

This paper compares the conventional exhaust system with a supply-exhaust system with regard to the possible degree of control of the air exchange in the individual rooms. Ventilation efficiency and air exchange efficiency are defined and some examples show the local concentration, mean ventilation efficiency and mean air exchange efficiency for some simple ventilation schemes. Exhaust systems require a very tight building with small make up air openings. The ability of the different systems to avoid leakage out from the building of indoor air is also compared. The calculations indicate that the exhaust system can give small duration of indoor air. The supply-exhaust system gives a greater duration, but the flow can be considerably reduced if the supply air flow rate is reduced. An extremely low velocity exhaust system will also reduce the unwanted outflow.