Where a reliable and positive flow of air for ventilation is required, fans can be installed in a building to extract stale air, which is immediately replaced by fresh air from the outside flowing through gaps around window and door frames, or through purposely designed grilles. Fans in more elaborate ventilation schemes are connected to systems of ductwork for better air distribution throughout
a building. Separate duct systems can be installed to pull away stale air from those used to distribute clean air. Often such systems are coupled with heating and cooling plant in such a way that the clean air is distributed at the selected temperature for thermal comfort air-conditioning
Artificial ventilation systems are usually employed for internally located rooms; crowded rooms where natural ventilation is insufficient; special rooms needing closely controlled humidity and/or freedom from any dust (e.g. computer rooms and operating theatres); and where polluted air is required to be either removed or prevented from entering internal spaces.