As air gets warmer it becomes less dense and so more buoyant. This means that warm air has a tendency to rise.
This effect can be used to naturally ventilate buildings. Cooler outside air is drawn into buildings at a lower level, it is warmed by sources of heat within the building (such as people, equipment, heating and solar gain), and then rises through the building to vent out at a higher level.
A positive pressure area is created at the top of a building and negative pressure area at the bottom. This process can take place without mechanical assistance, simply by introducing openings at the bottom and the top of buildings. It is known as the stack effect or stack ventilation.
Stack ventilation can be particularly effective as a means of naturally ventilating tall buildings that include vertical spaces which rise throughout their height, for example buildings with central atriums. This can be useful in deep buildings, where cross ventilation may not be sufficient to penetrate to spaces in the heart of the building.
The effectiveness of stack ventilation is influenced by:
- The effective area of openings.
- The height of the stack.
- The temperature difference between the bottom and the top of the stack.
- Pressure differences outside the building.
The stack effect is a relatively weak force in many buildings (although it can be strong enough to fly a kite in large buildings) and so it may be necessary to have large openings with minimal resistance.
The pressure will vary through the height of the building, with the outside pressure being higher than the inside pressure at the bottom of the building, but the inside pressure being higher at the top. As a consequence, there will be a neutral plane, which is the level at which the internal and external pressure are equal. Above the neutral plane the internal air pressure will be positive and so air will tend to exhaust to the outside. Below the neutral plane, the internal air pressure will be negative and air will tend to be drawn into the building. This requires careful design to ensure that the neutral plane is above the spaces that need to be ventilated from the outside.
Stack ventilation is not appropriate for all building types, and as the stack effect requires that the internal temperature is higher than the outside temperature, it may not always provide a sufficient cooling effect by itself and additional mechanical cooling may be necessary. In particular, rooms adjoining the warmer part of the stack may experience poor ventilation and unwanted heat gains. In addition, there may be conflicts between the requirement for large unrestricted openings between the outside and the heart of the building, and requirements for security, privacy, noise control, fire compartmentation and so on.
Designing natural ventilation can become extremely complex because of the interaction between cross ventilation and the stack effect as well as complex building geometries and complex distribution of openings. This can require computer analysis.
NB: Approved document F, Ventilation, defines 'passive stack ventilation' (PSV) as, ‘…a ventilation system using ducts from terminals in the ceiling of rooms to terminals on the roof that extract air to the outside by a combination of the natural stack effect and the pressure effects of wind passing over the roof of the building.’
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