Over the past 50 years the use of air conditioning in non-domestic buildings has become a norm and an indicator of status. Today the rediscovering of the natural ventilation is a part of rediscovering the buildings' energy efficiency, or maybe even a part of a wider approach, which is a desire to be closer to nature. The main task of all ventilation systems is to maintain an appropriate indoor air quality and to improve the indoor environment. Natural ventilation systems could do the above using less energy than mechanical systems. However, it requires also the implementation of other passive measures. The most important of them are: the reduction of the harmful air contaminants, the control of heat gains, the exposition of the building's thermal mass and utilisation of the night cooling. Because of energy efficiency and thermal comfort reasons, in temperate climate ventilation systems have to work according to at least three scenarios: spring/autumn, winter and summer. The thermal comfort parameters in naturally ventilated buildings are usually more variable than in air-conditioned ones, what does not mean that the occupants will experience thermal discomfort. Therefore, thermal comfort in passively ventilated buildings should be evaluated according to the adaptive comfort standard, appropriated for the naturally ventilated buildings. The natural ventilation has its limits and probably not all buildings can be ventilated naturally. From the energy efficiency and thermal comfort reasons, implementing the mixed mode systems is sometimes more feasible. However, the real reason why the full potential of natural ventilation could not be explored is very often the lack of confidence in relying exclusively on it.
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