Centralised air systems - heating and cooling is carried out in a central plant room and conveyed by ductwork. Typical examples are:
- Constant volume (CV) systems : relatively low cost and easy to commission but unable to provide adequate zone control. Separate systems may be required for different zones, increasing capital costs and plant space. Mainly used to provide fresh air in partially centralised air/water systems.
- Variable air volume (VAV) systems : provide zone control by varying the quantity (volume) of air at a constant temperature. Can reduce costs by operating supply and extract fan(s) at reduced speeds (and therefore volume) for much of the year. Can be expensive and complex, requiring careful commissioning. Mainly found in buildings with year-round cooling demands.
- Displacement ventilation systems : provide low level cool air from a central plant at low speeds but unable to remove large heat gains. Mainly used to provide ventilation in conjunction with chilled beams/ceilings.
Partially centralised air/water systems - centrally cooled or heated air is further heated or cooled on entry to rooms. Typical examples are:
- Centralised air systems : a central air supply provided either at constant or variable volume which is further heated or cooled to individual room requirements by additional heating or cooling coils (batteries). This provides greater control in constant volume systems and can also benefit variable systems in areas with widely varying loads.
- Four-pipe fan coil units : typically comprise a fan, heating and cooling coils and air filters. Fresh air and heated or chilled water are provided to each unit and the air is heated or cooled just prior to entry into a space. Fan coil units are mounted either at the building perimeter or at high level, often above a suspended ceiling. Units must be carefully selected to avoid noise problems and require a lot of maintenance.
- Chilled ceilings and beams : consist of ceiling mounted panels of pipes that are cooled by cool (14-17°C) water. Cold air sinks through a space or is radiated downwards. Using water saves on fan energy, however, these should not be used in areas where a lot of moisture is generated as condensation may become a problem. Water temperatures are warmer than standard water-based systems, reducing system cooling capacity. This offers potential for other sources of chilled water e.g. rivers, cooling towers or ground water.
- Room based heat pumps : individual and independent units linked together by a piped water circuit that runs around a building. Can heat or cool the immediate area but are only effective in buildings with zones that have different heating and cooling periods. The system also requires a separate ventilation system.
Local systems – all operations are performed locally. Typical examples are:
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Split systems : sold as a complete package so quick to install with minimum disruption to occupants. Have two basic components - indoor room cooling unit and an outdoor refrigeration (i.e. condensing) unit linked by pipes transporting refrigerant. Some systems can also provide heating by reversing the process. Typically found in shops, restaurants and office areas and are only suitable for relatively small spaces. Require servicing by specialist maintenance technicians
- Variable refrigerant flow (VRF) systems : sophisticated split systems providing heating or cooling (using refrigerants or water) on an individual basis. Useful in office blocks, hotels and large retail stores etc, which may require cooling in some areas and heating in others. Outside condensing units of a VRF installation can be widely spread making them particularly useful for refurbishment projects. Require installation and servicing by specialist maintenance technicians.