Natural supply and mechanical removal (system C)
With a system for natural air supply and mechanical air removal (Figure 1) used air is mechanically extracted from several areas in the home (at least in the kitchen, toilet and bathroom). This creates underpressure, letting fresh air from outside enter the home naturally through openings in the facade (usually through grids). By means of air transfer facilities (usually a slit under the door), the supplied air can flow to the rooms from where the air is removed. Most modern systems are equipped with a multi-position switch, with which residents are able to adjust the air discharge to a higher or lower level and thus determine the amount of air supply and discharge (rate of flow). With this ventilation system, the supplied air directly enters the home through the grids in the facade. When it is cold outside and windy, this can cause drafts. To reduce the risk of draft, self-regulating grids (wind pressure regulated) are available, which automatically close when there is wind on the facade.
Basic diagram of a natural ventilation system with natural supply and mechanical removal (blue / green is air supply, brown is air removal).