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How Air Conditioning Works

Refrigeration Circuit

There are just four processes in the refrigeration cycle. Refrigerant simply flows through our air conditioning system changing state at various points.

The outdoor ‘condensing’ unit utilises a compressor to pump refrigerant around the system. The gas entering the compressor is at a low pressure

The compressor compresses the gas, thus becoming high pressure and the temperature is increased. This gas flows into the condensing coil.

In the condensing coil this high pressure, high temperature gas dissipates its heat (with the aid of the condensing fan) and is turned into a high pressure liquid.

This high pressure liquid passes through an expansion valve. An expansion valve reduces the pressure of the liquid producing a cold low pressure liquid.

This cold, low pressure liquid flows through the liquid line to the indoor evaporator where it ‘evaporates’, absorbing the heat from within the room. Air is blown over the coil to aid transfer.

The liquid reduces its pressure becoming a low pressure gas. This low pressure gas flows back to the outdoor condensing unit.

Cooling Cycle

In the cooling cycle the outdoor unit can be referred to as the condenser, while the indoor coil can be referred to as the evaporator.

The evaporator (indoor unit) can absorb heat from within the room and the condenser (outdoor unit) dissipates the heat to the outside air.

Heating Cycle

In the heating cycle the outdoor unit can be referred to as the evaporator, while the indoor coil can be referred to as the condenser.

Although the outside temperature can drop to -10°C, the air still has heat in it and the outdoor coil absorbs heat from the external air.

Heat Pumps

Heat pumps do not have additional pumps ! Simply put a heat pump system can send refrigerant both ways around the system, the compressor still does all the work.

Once a 'modern' system is installed (such as Mitsubishi, Daikin or Toshiba) reversing valves and the subsequent controls can’t be retro fitted (economically at least).


A common misconception is that air is transferred from outside to in, this is not the case with any ‘split’ type system, the only exceptions are ducted systems with Dx coils within them.

Another misconception is that how can you have greater efficiency than 100%, some systems have a Coefficient (COP) of performance greater than 4:1 ?

Remember it’s the refrigerant properties that provide the heating and cooling not the compressor. The electrical load is just required for the compressor.

Phoenix Air Con Ltd - Installer of air conditioning units & industrial & commercial heating systems - installation & maintenance service company - Cwmbran, Newport, Pontypool, Eastern Valleys, Gwent & South Wales

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