There are two ways to ventilate or cool buildings: actively or passively. Active ventilation/cooling refers to systems where mechanical components or other energy-consuming components (such as air-conditioning systems) are used. Passive ventilation/cooling is a technology or design feature used to ventilate/ cool buildings with no energy consumption (e.g. natural ventilation by openable windows).
Natural ventilation uses natural forces to exchange the air in a building. The driving forces are wind and temperature differences,
Mechanical ventilation systems use electric fans to direct the airflow in the building. Mechanical ventilation can provide a constant air change rate independently of external weather conditions, but it uses electricity and usually cannot change the ventilation rate as the need changes over the day and year.
It is becoming a standard solution in many North European countries for newbuilt houses to be provided with mechanical heat recovery ventilation in order to meet current energy requirements. This is a very energy efficient solution for the heating (winter) season. However, in the summer season, electricity for running of fans can be saved by using natural ventilation. Systems shifting between natural ventilation and mechanical ventilation are called hybrid ventilation systems.