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<B>Centralised air system</b>
Centralised systems have all the heating and cooling carried out in a central plant room and conveyed to the occupied space by ductwork.
They are based around a packaged air-handling unit, which consists of a fan and combinations of heating and cooling coils, filters, humidifiers and control dampers. They may also include packaged heat pumps, and an exhaust fan or the facility to recycle exhaust air back into the building. The air-handling unit is normally located in a plant room with chillers and boilers nearby. They can be configured to serve several different types of distribution system.
Constant volume single-zone systems are simple, relatively cheap and easy to commission, but cannot provide adequate control for areas with different and simultaneous heating and cooling needs. Several separate systems may be required to serve different zones, increasing capital costs and plant room space.
Variable air volume (VAV) systems address the problem of zones with different requirements by varying the quantity of air supplied to each space. The air is supplied at a constant temperature through thermostatically controlled damper units (called VAV boxes). The volume of air and hence the amount of cooling is varied to meet the requirements of each zone. The use of VAV boxes means that some maintenance work on these boxes (which are usually located in the ceiling voids) must be carried out in the occupied space.
Within the VAV concept, the designer has several options, including fans to re-circulate air and by-passing air that is not required back to the extract fan. This can introduce noise and add to the maintenance requirements.
VAV systems are primarily for buildings with a year-round cooling demand. They are normally smaller than the equivalent multi-zone system because the design does not allow for full cooling simultaneously in all zones.
Dual duct systems can incorporate either constant volume or VAV principles. As the name implies, two ducts are used, one carrying heated and one cool air to the space, where the air is mixed in a thermostatically controlled mixing box, usually mounted in the ceiling void. These systems give accurate control of the space temperature, but capital costs and space requirements are high because two sets of ductwork are required. In their constant volume form, dual duct systems will often mix air that has been heated with air that has been cooled.

<B>Partially centralised air/water systems</b>
These systems use centrally cooled or heated air, which is further heated or cooled at entry to the space.
Air/water systems use a central air-handling unit, as described above, but further conditioning in the room is by local control. Partially centralised multi-zone and VAV systems allow free cooling by damper control at the central air-handling unit.
Centralised air systems with re-heat of both constant volume and VAV types are available where the central air supply is further heated or cooled to room requirements by additional heating or cooling coils. In constant volume systems this improves controllability and the ductwork can be configured to serve rooms with very different requirements. In VAV systems, reheating coils are usually provided locally within the VAV box. This has the disadvantage that hot water must be piped to the VAV boxes with potential for leaks in the occupied areas. However, they provide good control for areas with widely varying loads.
Fan coil units use air from the central air-handling unit, the air being moved by a local fan. Fan coil units can be perimeter or ceiling mounted, and are very quiet when using the latest fan designs. However, a large number of older units can create maintenance and noise problems.
The units usually have a fresh air supply from the air-handling unit, which provides minimum fresh air requirements at a much lower volume than an equivalent multi-zone system. Central plant room and ductwork space requirements are low; the room VAV units are usually mounted in the ceiling space.

<B>Local systems</b>
Local systems, where all operations are carried out locally, provide "comfort cooling" rather than full air-conditioning. Such systems do not provide fresh air supply, humidity control or heating. They are characterised by one unit per conditioned zone when only small parts of a building require conditioning. Examples of types of units are "packaged split units" and "variable refrigerant flow rate systems".
In packaged units, the room-mounted part of the unit resembles a fan coil unit, but cooling is provided by refrigerant rather than chilled water. The refrigeration part of the unit can be located away from the occupied area. Some units offer variable speed compressors and modulating temperature control with remotely mounted sensors. Multi-split package systems are also an option where several coolers are connected to one refrigeration unit.
Variable refrigerant flow-rate systems are a special case of multi-split package heat pumps. Several room coolers are connected directly to a single outdoor refrigeration unit. The refrigerant flow rate can be varied using a variable speed compressor in response to changes in cooling requirements. In some versions, the indoor units may operate in heating or cooling modes independently of the other units. These systems can be advantageous where no plant room is available and where a number of zones have different cooling and heating requirements. They offer greater flexibility, but maintenance costs can be significant.

<B>Factors affecting durability</b>
Installation standards need to be of a high standard to ensure good airflow with minimal resistance. Dust-laden air will mean increased frequency of filter replacement. Contaminated or corrosive air (for example, in a salt-laden environment) will reduce the service life of components if not designed to cope.
Regular inspection and maintenance by skilled personnel is essential. Wet systems need appropriate water treatment to avoid internal corrosion. Systems must be designed to meet the intended load – if operated continuously at or above their maximum capacity, the service life will be reduced.

<B>Durability tips</b>

• Ensure a high standard of design, specification and installation
• The system should be fully tested and commissioned before handover
• A proper maintenance and cleaning regime should be instituted and monitored
• Any signs of corrosion must be treated quickly
• Routine ductwork inspection should be carried out and where appropriate the system cleaned
• Humidification equipment should be monitored for Legionnaires' disease.

<B>Reasons for failure</b>

• Fans: motor coupling or drive belts loose or broken, dirt build-up on fan blades, fan blades damaged or corroded
• Motors: loose terminals, blocked airways or windings burned out
• Terminal units: airways blocked, filter needing replacement or motor failure
• Filters: blocked or needing replacement
• Dampers: corroded or incorrectly set
• Insulation: damaged
• Heating and cooling source: incorrect operation, inability to supply required capacity, or loss of performance
• Heating and cooling heat exchangers: corrosion or blockage due to dirt or debris build-up
• Controls: incorrect settings, inadequate commissioning, valve or sensor failure
• Blocked air inlets: if not a blockage, it could be that the filter needs replacing
• Water treatment: corrosion, internal build-up of deposits leading to inadequate heat exchange
• Air leakage: ductwork and components badly installed, damaged or corroded
• Fire dampers in ductwork not functioning due to corrosion or damage.