Mechanical ventilation of buildings
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Ventilation is necessary in buildings to remove ‘stale’ air and replace it with ‘fresh’ air:
- Helping to moderate internal temperatures.
- Replenishing oxygen.
- Reducing the accumulation of moisture, odours, bacteria, dust, carbon dioxide, smoke and other contaminants that can build up during occupied periods.
- Creating 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. For more information see Stack effect and Cross ventilation.
Natural ventilation is generally preferable to mechanical ventilation as it will typically have lower capital, operational and maintenance costs. However there are a range of circumstances in which natural ventilation may not be possible:
- 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.
- Privacy or security requirements prevent windows from being opened.
- Internal partitions block air paths.
- The density of occupation, equipment, lighting and so on creates very high heat loads or high levels of contaminants.
Some of these issues can be avoided or mitigated by careful design, and mixed mode or assisted ventilation might be possible, where natural ventilation is augmented by mechanical distribution or extract.
Where mechanical ventilation is necessary it can be:
- A circulation system such as a ceiling fan, which creates internal air movement, but does not introduce ‘fresh’ air.
- A pressure system, in which ‘fresh’ outside air is blown into the building by inlet fans, creating a higher internal pressure than the outside air.
- A vacuum system, in which ‘stale’ internal air is extracted from the building by an exhaust fan, creating a lower pressure inside the building than the outside air.
- A balanced system that uses both inlet and extract fans, maintaining the internal air pressure at a similar level to the outside air and so reducing air infiltration and draughts.
- A local exhaust system that extracts local sources of heat or contaminants at their source, such as cooker hoods, fume cupboards and so on.
In commercial developments, mechanical ventilation is typically driven by air handling units (AHU) connected to ductwork within the building that supplies air to and extracts air from the interior. Typically they comprise an insulated box that forms the housing for; filter racks or chambers, a fan (or blower), and sometimes heating elements, cooling elements, sound attenuators and dampers. In some situations, such as in swimming pools, air handling units might include dehumidification. See Air handling units for more information.
Extracting internal air and replacing it with outside air can increase the need for heating and cooling. This can be reduced by re-circulating a proportion of internal air with the fresh outside air, or by heat recovery ventilation (HRV) that recovers heat from extract air to pre-heat incoming fresh air using counter-flow heat exchangers.
The design of mechanical ventilation systems is generally a specialist task, undertaken by a building services engineer. Whilst there are standards and rules of thumb that can be used to determine air flow rates for straight-forward situations, when mechanical ventilation is combined with heating, cooling, humidity control and the interaction with natural ventilation, thermal mass and solar gain, the situation can quickly become very complicated. This, along with additional complications, such as the noise generated by fans, and the impact of ductwork on acoustic separation means it is vital building services are considered at the outset of the building design process, and not seen as an add-on.
Mechanical ventilation may be controlled by a building management system (BMS) to maximise occupant comfort and minimise energy consumption. Regular inspection and maintenance is necessary to ensure that systems are operating optimally and that occupants understand how systems are operated.
 Related articles on Designing Buildings Wiki
- Air conditioning.
- Air infiltration testing.
- Building management systems.
- Building services.
- Chilled beam.
- Computational fluid dynamics.
- Constant air volume.
- Cross ventilation.
- Dew point.
- Displacement ventilation.
- Domestic ventilation systems performance.
- Electrical control systems.
- Heat recovery.
- Interstitial condensation.
- Mechanical, electrical and plumbing MEP.
- Natural ventilation.
- Passive building design.
- Solar chimney.
- Stack effect.
- Thermal comfort.
- Variable air volume VAV.
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