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Stack ventilation is a natural ventilation method that relies on a spontaneous circulation of air caused by the thermal stack effect. This naturally occurring phenomenon causes hot air to rise in an enclosed space and seek an exit. When hot air leaves the space, it creates a partial vacuum which draws in cooler air from the outside atmosphere to replace it. This cycle will continuously introduce cooler air into the space, thereby effectively regulating the temperature and ensuring a constant supply of fresh air. This system of heat and air exchange is most effective in buildings with dedicated stack ventilation systems and where large differences in exterior and interior temperatures encourage the stack effect.
The natural phenomenon of hot air rising above cooler air is fairly well known and forms the basis for all stack ventilation systems. When air is heated, the molecules move further apart, thus making the air less dense and more buoyant. The body or column of air in any enclosed space will always be hotter towards the top of the space due to this effect. Stack ventilation systems use this naturally occurring movement to expel hot air and draw fresh, cooler air into a space. These systems are particularly efficient because they require very few, if any, mechanical components to operate correctly.
Dedicated stack air exchange systems will typically feature a series of hot air vents at roof level and cold air inlets along the lower parts of the exterior walls. The hot air outlet vents should ideally be located as high as possible; the stack effect becomes more pronounced when the distance between inlet and outlet points increases. Larger, multi-floor buildings usually have a central hot air duct which leads to the roof of the structure with hot air vents at ceiling level on each floor. The cool air inlets are then usually fed by air which is drawn up a separate cool air duct by fans. This duct will have a set of intake vents at ground level.
When the air in the interior space starts to heat up, it will naturally rise to the upper regions of the space and exit from the hot air vents. As it does so, the low pressure vacuum left in its wake draws fresh, cooler air into the area through the cold air vents. This cycle sets up a constant cross flow ventilation pattern that keeps the area supplied with fresh air. Since this cycle relies on a difference in temperature between the outside and inside air columns, stack ventilation systems are particularly effective during winter periods.
Stack ventilation has a number of advantages and disadvantages which have to be considered when choosing a ventilation system. The main benefits of stack ventilation systems include the flexibility in vent placement, non-reliance on wind patterns, stable air flow, and sustainability. The disadvantages of this type of system are lower air flow force when compared to wind ventilation, reliance on temperature differentials, and design limitations regarding ceiling heights. Disadvantages notwithstanding, these systems are generally effective, low cost, and environmentally friendly air exchange options.