A ventilation system is a mechanical structure of connected devices that controls airflow within confined spaces, commonly homes and offices. Its main function is to introduce a constant supply of fresh air, usually from the outside, while channeling stale air back out.
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Ventilation is necessary in buildings to remove 'stale' air and replace it with 'fresh' air.
This helps to:
- Moderate internal temperatures.
- Reduce the accumulation of moisture, odours and other gases that can build up during occupied periods.
- Create air movement which improves the comfort of occupants.
Very broadly, ventilation in buildings can be classified as 'natural' or 'mechanical'.
- Mechanical (or 'forced') ventilation tends to be driven by fans.
- Natural ventilation is driven by 'natural' pressure differences from one part of the building to another. Natural ventilation can be wind driven, or buoyancy driven.
Whilst natural ventilation may be preferable, mechanical ventilation may be necessary where:
- The building is too deep to ventilate from the perimeter.
- Local air quality is poor, for example if a building is next to a busy road.
- Local noise levels mean that windows cannot be opened.
- The local urban structure is very dense and shelters the building from the wind.
- Air cooling or air conditioning systems mean that windows cannot be opened.
- Privacy or security requirements prevent windows being opened.
- Internal partitions block air paths.
- The creation of draughts adjacent to openings.
NB: Some of these issues can be avoided or mitigated by careful siting and design of buildings.
'Mixed-mode' ventilation uses both natural and mechanical ventilation, for example, allowing the opening of windows, but also providing a mechanical air distribution system.
The term 'assisted ventilation' typically refers to systems where fresh air enters a building through windows or other openings, but is extracted by continuously running fans.
'Trickle Ventilation', 'slot ventilators' or 'background' ventilation can be necessary in modern buildings, (which tend to be designed to be almost completely sealed from the outside to reduce heat loss or gain) so that problems such as condensation, are avoided when openings are closed.
This tendency to 'seal' modern buildings can also adversely affect occupant comfort, as generally, occupants feel more comfortable if there is some air movement (as long as draughts are not created). This situation can be mitigated by heat recovery ventilation (HRV). This permits increased ventilation rates by recovering heat from extract air and using it to pre-heat incoming fresh air using counter-flow heat exchangers. Heat recovery is increasingly common in mechanical ventilation systems. It is also possible, although complicated with some natural ventilation systems.
Ventilation systems may also include heating, cooling, filtration and humidity control. The acronym HVAC refers to Heating Ventilation and Air Conditioning. The phrase 'air conditioning' refers to the process of conditioning the temperature and humidity (and sometimes the quality) of air before using it to ventilate a building. Air conditioning and cooling and are not the same, although the terms are often used synonymously by non-professionals.
Rates of ventilation in buildings can be expressed in terms of air change rates(the number of times that the volume of air in a space is changed per hour) or litres per second. The ventilation rate will be determined by the type and size of space and the way it is occupied (for example, the number of occupants, sources of heat, moisture, odour, contaminants, and so on).
Whilst there are simple 'rules of thumb' that can be used to design straight-forward ventilation systems, more complex systems may require analysis using environmental design software. Modelling air flow patterns is particularly complex requiring the use of computational fluid dynamics (CFD) software. This is complicated further by the interaction of ventilation systems with thermal mass solar radiation, and so on. Whilst there are software packages that can be used for this sort of analysis, the results they produce are very dependent on the way that models are set up, and this requires a great deal of expertise and experience.