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Last edited 03 Sep 2019
Heating ventilation and air conditioning HVAC
Internal air quality can be maintained by a combination of introducing 'fresh' air into the building, extracting 'stale air' and by filtration. Ventilation may be natural, mechanical, or mixed mode (a hybrid system). See Ventilation for more information.
Internal temperatures can be regulated by heating and cooling. Typically, this is achieved by heated water (or sometimes steam) and chilled water that is generated by boilers and chillers and then used in heating coils and cooling coils as part of the ventilation system. Alternatively, hot water may be used to supply systems such as radiators, underfloor heating and so on.
Humidity can be regulated by ventilation, dehumidification and humidification. Dehumidification is often provided alongside cooling as cooling air reduces the amount of moisture air is able to 'hold', resulting in condensation. 'Close' humidity control (to within 10%) can involve cooling and dehumidification, then re-heating and re-humidification.
Very broadly, HVAC systems can be centralised in a building, or local to the space they are serving, or a combination of both (for example, local air handling units supplied by centrally-generated cooling). They may also be connected to a wider district heating or cooling network.
They may be integrated, with heating, ventilation and air conditioning provided by a single system, for example, air handling units connected to ductwork, or they may be a combination of separate systems, for example mechanical ventilation with radiators for heating and local comfort cooling units.
They may also include passive (or 'natural') systems such as natural ventilation.
In mechanically ventilated commercial developments, HVAC is often provided by air handling units (AHU) connected to ductwork that supplies air to and extracts air from internal spaces. Air handling units typically comprise an insulated box that might include some, or all of the following components; filter racks or chambers, a fan (or blower), heating elements, cooling elements, dehumidification, sound attenuators and dampers. Air handling units that consist of only a fan and a heating or cooling element, located within the space they are serving, may be referred to as fan coil units (FCU). 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, or by heat recovery ventilation (HRV) that recovers heat from extract air and uses it to pre-heat incoming fresh air.
It is important that all aspects of HVAC systems are considered together during the design process, even where involve independent systems. This is because of the interaction between heating, cooling, humidity control and ventilation. This is particularly complicated when other elements of environmental behaviour are considered such as solar gain, natural ventilation, thermal mass, and so on.
The design of HVAC systems is generally a specialist task, undertaken by a building services engineer, and because of its interaction with other elements of the building it is important that it is considered from the outset, as a fundamental part of the design process, and not an 'add on' at the end.
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