Natural ventilation: Traditionally, ventilation needs have been met by ‘natural’ ventilation in which the flow process is driven by wind and temperature. In mild climates, design has often relied on no more than the natural porosity of the building, combined with window opening. In colder climates, natural ventilation designs tend to be more specific and incorporate carefully sized air inlets combined with passive ventilation stacks. Other climates might take advantage of a prevailing wind to drive the ventilation process.

The main drawback of natural ventilation is lack of control, in which unreliable driving forces can result in periods of inadequate ventilation, followed by periods of over ventilation and excessive energy waste. Good design can provide some measure of flow control but normally it is necessary for the occupant to adjust ventilation openings to suit demand. Despite the difficulty of control, natural ventilation is still relied upon to meet the need for fresh air in many types of building throughout the world.

Mechanical ventilation: In principle, the shortcomings of natural ventilation can be overcome by mechanical ventilation. These systems are capable of providing a controlled rate of air change and respond to the varying needs of occupants and pollutant loads, irrespective of the vagaries of climate. Some systems enable incoming supply air to be filtered while others have provision for heat recovery from the exhaust air stream. In some countries, especially in parts of Canada and Scandinavia, mechanical systems are being incorporated into virtually all new building construction and included in many building refurbishment programmes. In milder climate, however, the potential advantages of mechanical ventilation, especially for smaller buildings, can often be outweighed by installation and operational cost, maintenance needs and inadequate return from heat recovery. 
Regardless of climate, mechanical ventilation is often essential in large, deep plan office buildings where fresh air must penetrate to the centre of the building and high heat gains can cause over heating.

Several configurations of mechanical ventilation are possible with each having a specific range of applications. The basic options are:

  • supply ventilation,
  • extract (or exhaust) ventilation,
  • balanced supply extract systems