The performance of natural, mechanical and hybrid ventilation systems was monitored, using the tracer gas decay method, in three typical apartment buildings located in two street canyons, during summer period 2002 in Athens. The multi-zone methodology has been adopted based on the mass balance of two tracer gases (N_2O and SF_6) to define the air change rates. The air-exchange efficiency was determined for different ventilation systems, on the basis of the room mean age of air. In spite of the canyon effect, appreciable ventilation rates can be obtained with natural ventilation, especially when cross ventilation with two or more windows is measured. For single-sided ventilation or under calm conditions hybrid ventilation has only a slight advantage over natural, in terms of air exchange rates.
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