Most people think of ventilating a building or room only when they feel uncomfortably hot. Until they feel the discomfort caused by accumulated hot air, the idea of ventilation seldom occurs. There are other reasons for ventilating a building which are less prevalent.
The word "ventilate" is defined in the dictionary as "to let fresh air into (a room or building)" and a "ventilator" is defined as "an opening or device, such as a fan, used to let fresh air into a room or building."
A room or building needs ventilation for one or more of the following reasons:
- Trapped air accumulates heat
- Trapped air becomes contaminated with toxic fumes, particles and odours
- Trapped air becomes saturated with water vapour
- Trapped air becomes depleted of oxygen causing stuffiness
- Smoke and fumes from fire choke inhabitants and cause the fire to spread
The most cost effective form of ventilation would be a hole in the roof, as this would allow the air to escape naturally. However this is generally impractical as a hole will allow dust, rain etc. to enter the building.
Mechanical vs. Natural Ventilation
The manner in which a building is ventilated can take one of two forms; mechanical or natural. Mechanical ventilation employs power such as electricity, gas, coal, oil etc. and natural ventilation harnesses nature's forces.
Natural ventilation will solve, on average, 75% of a given ventilation problem. Solving the remaining 25% usually turns out to be too expensive. A turbine ventilator moves air constantly, causing trapped air to be replaced with cleaner, cooler air from outside.
Natural ventilation is driven by pressure differences, like the wind, air density differences due to temperature differences between indoor and outdoor, and the operation of equipment generating heat inside the building.
Temperature differences between indoor and outdoor cause air density differences, and therefore pressure differences. The taller the building and the less internal resistance to airflow, the stronger the stack effect.