Natural ventilation is based on
the principle that the difference in pressure between the air inlet
and exhaust air vents in a building allows natural air exchange
to take place. The pressure differences arise due to:
- the buoyancy forces in the building caused by differences in
density between the air in the building and the outside air, and
the difference in height between the air inlet and exhaust air
- the air currents in the building.
Accordingly, the volume of air
flowing through the building depends on the surplus heat in the
building – caused by the convection heat that is emitted into
the room air (internal cooling load), the external cooling load
(transmission through insolation), wind speed, wind direction and
The volume of air can be controlled and maintained within fixed
parameters according to this principle.
Fig. 1 and Fig. 2 show a simplified natural ventilation scheme.
The neutral plane is located where internal and external pressure
is equal. Vents are not effective at this point. Above the neutral
plane, internal pressure is higher than external pressure, and this
is where the exhaust air vents are located. Underneath the neutral
plane, external pressure is higher than internal pressure, which
is why the fresh air vents are located here.
When designing a natural ventilation system, it is necessary to
predetermine the height of the neutral plane and ascertain the proportion
of pressure difference that is available to overcome flow losses
in the air inlet vents and the proportion that will be used by the
exhaust air vents.
It is always necessary to calculate the position of the neutral
You will find explanations of technical terms in our 'Industrial
Ventilation' compendium, which is available as a PDF file in our