Also known as a System 1 ventilation system. Extract fans are used to serve moist spaces such as kitchens, toilets, and utility rooms for fast intermittent air extraction in this form of decentralised ventilation scheme.
The substitute air is supplied by a background ventilators. The phrase background ventilators relates to tiny air vents which enable air into a building, for example trickle vents in the windows or air vents in internal walls. Manual or automatic control can enable this system.