An accumulator temporarily collects refrigerant to prevent back-flow into the compressor.

Air Handling Unit (AHU)

In a ducted system, the central AHU contains the heating and cooling elements, blower, filters, humidifier, dampers and other regulating equipment. On a roof, it’s called an RTU (rooftop unit).


The chiller contains the primary cooling medium (refrigerant) which removes heat from a secondary medium (such as water) through cycles of compression and evaporation. The secondary medium then cools the air by passing through coils in other units around the building. Chillers are cooled by fan-driven external air, or by water circulating through a cooling tower.


Compression pushes refrigerant through its closed cooling loop. Variations include screw and scroll compressors.


As opposed to the “evaporator” this is the hot side of an air-conditioner where the refrigerant condenses and heat is extracted either to disperse it in the environment (for example in a cooling tower) or to recycle it for other purposes.


CAV systems supply a “Constant Air Volume” but vary its temperature. Conversely, “Variable Air Volume” systems control temperature by varying the airflow and save energy by running slower when demand is low.


Dampers are placed at various points in ducts to control the air that can flow through. They can be manual but are usually automatically controlled to ensure the most efficient and economical flow of air. When composed of angled slats they are called “louvres”. Similar devices placed where ducts branch are more correctly called “turning vanes”.


A diffuser splits airstreams in multiple directions to produce a balanced airflow. They are usually located at outlet points. Outlet grilles can also be designed to direct air in a particular direction.


Comparing the heat content (enthalpy) of air inside and outside a building allows economisers to vary the intake of environmental air to maximise energy efficiency.

Enhanced Capital Allowance (ECA)

The ECA is a government scheme that rewards businesses with tax relief for investing in new energy-saving equipment.

European Seasonal Energy Efficiency Ratio (ESEER)

This is a performance rating for products such as chillers. It is their cooling output divided by electricity input under different seasonal conditions. If you can capture and reuse energy, for example, use heat from computers to power the cooling system, the EER is amended by the ERE (energy reuse effectiveness).


The evaporator is the component of an air conditioner or a heat pump where the cooling takes place. It often takes the form of a coil. Heat is drawn out of the air passing over it.

Fan Coil Unit (FCU)

An FCU is a small indoor terminal unit (TU) that controls the temperature in a single room. FCUs contain only a fan, valve and cooling coil linked to a chilled water supply. They may be freestanding or mounted on walls or ceilings.

Filters and Purifiers

Filters remove particulates from the airflow, keeping the ducts clear and the air safer to breathe. Basic filters are usually made of polyester fibres or plastic mesh. HEPA filters (often paper-based) are more effective and suppress allergens. Charcoal filters can also remove many gaseous pollutants. Air purification can also be achieved with units that incinerate small particles, including bacteria or trap them electrostatically.

Heat Exchanger

A heat exchanger transfers heat from a hot medium to a cold one. They are used to recycle waste energy to power or supplement other parts of the system.

Heat Pump

A heat pump is a compressor or other device that moves heat from one area to another. The direction and rate of flow change as the conditions change in each space. Frequently they can heat one space while cooling another.

Monoblock Air Conditioners

These type of air conditioners are self-contained devices that expel air outside through a pipe. They are easy to install but not as efficient as split and ducted systems.

Outdoor Unit

In non-ducted air conditioners, the outdoor “condenser” or “heat-exchanger” unit contains a compressor, fan, circuit board and heat exchange coil. It pumps refrigerant to and from one or more indoor units.


Refrigerants are chemicals that readily evaporate and condense within a useful temperature range. Potential refrigerants include ammonia and carbon dioxide, but hydrocarbons are the most cost-effective. Examples include propane (R290), isobutane (R600a), tetrafluoroethane (R134a) and hydrofluoroolefins (e.g. R1234ze). Older fluorine-based refrigerants (F-gases) such as R22 and R12 cause climate change and damage to the ozone layer and are being superseded.

Split System

A split system uses an outdoor condenser and an indoor terminal unit. They avoid the need for ductwork. In a multi-split system, a single outdoor condenser links to multiple indoor units in different rooms or apartments.


Sensors that report air temperature to the main unit where it’s used to switch the supply of cooling or heating. Some allow the target temperature to be set locally, others don’t.


Independent control of areas with different target temperatures or different starting conditions enables energy saving. Each zone has its own “unitary controller” such as a thermostat or airflow damper. Building regulations often require zoning of large commercial buildings.