If a cooker hood or other extractor device creates a little negative pressure, this will automatically pull more air through the ventilation windows. The conditioning effect will diminish, corresponding to a situation in which the occupant opens a window to allow fresh air inside the house.

The number of ventilation windows and vents needed varies depends on the property but is always based on a calculated 12-15 pascals of differential pressure. At this pressure difference, optimal conditioning is achieved.