Mechanical ventilation systems use fans to supply fresh or tempered air to a space and to extract stale air from it. This is in contrast to natural ventilation systems that simply allow air to enter from the outside via a window or grille.
Mechanical ventilation systems consume more energy than natural ventilation systems; however, they also enable the use of heat recovery and are thus much less wasteful in terms of energy.
The energy efficiency of a mechanical ventilation system is affected by factors such as the power consumption of the fan and the inherent pressure losses through the distribution ductwork. Resistance within a ventilation system requires increased fan power to move the required volumes of air.