Air conditioning is the process of cooling the ambient air in rooms/buildings and at the same time controlling the relative humidity by adding back the correct amount of moisture. This is to achieve a relative humidity of around 50%, which is generally considered to be most comfortable. When the air is cooled and no control of humidity takes place the correct term is actually ‘comfort cooling’ but this is more commonly referred to as air conditioning. Air conditioning / comfort cooling are normally achieved by use of refrigerant based systems which utilise two heat transfer coils. The coil, which dissipates the heat, is the condenser coil and the coil over which the air is cooled is the evaporator coil. The condenser can be installed outside the building and the evaporator unit is floor, wall or ceiling mounted inside the building.

A different type of cooler is the evaporative cooler – this is not air conditioning.
It is a fan with a moist filter across it. The filter is kept moist by a tank of water, which needs to be topped up regularly. Air passing through the filter evaporates the water from the filter and the latent heat of evaporation is taken from the air, so making it feel cool. The air is, however, moist and if these units are used in confined spaces then the humidity of the room will continue to rise and as it reaches near 100% evaporation will stop and the unit will stop working. These are best used in open, well-ventilated spaces where a cool breeze is needed and the air is continually vented to the outside.

There are different designs of air conditioner; most people in the UK are familiar with the "split system". These can be seen in many offices and server rooms where the evaporator is either wall mounted or a ceiling cassette.

Where buildings have central air conditioning systems they can be combined with heating systems and the fan coil units in the rooms can then either heat or cool the area as demanded by the control system or building management system. These systems can be very complex in design whereas split systems are relatively simple and ideal for retrofitting in buildings where no air conditioning has been installed from the outset.

Refrigerant based air conditioning systems often have the facility to operate in reverse. In this mode they are termed as a heat pump. Heat pumps take heat from outside and discharge heat via the coils in the indoor unit.

Whether in air conditioning or heat pump mode these types of systems typically have a coefficient of performance (CoP) of about 3. This means that a 3 KW air conditioner uses about 1 KW of power. Conversely a heat pump provides about 3 KW of heat while using about 1 KW of power.

Where the installation of an air conditioning system is not practical, or is not sensible due to short-term occupancy of the building, temporary air conditioners can be used. Air conditioner hire is common and these units can be split types or monobloc units.
Monobloc air conditioners have both evaporator and condenser coils in the same casing and discharge heat outside by exhausting hot air through a flexible duct, which is taken to a window or a vent in the wall. These air conditioning units will remove moisture from the air and so reduce humidity but they have no means of controlling humidity. Split air conditioners have the condenser coil contained in a housing inside the room and the separate evaporator is mounted externally and is connected with the refrigerant hoses.