1. Indoor unit – a fan blows hot room air over a heat-exchanging coil that contains cold refrigerant (the Evaporator). The cold refrigerant absorbs heat and cool air is blown into the room
  2. Pipework – refrigerant is moved through the system by a specialised pump called a compressor. Heat collected from the indoor unit is transferred to the external unit (the Condenser)
  3. External unit – compression heats up the refrigerant gas even further and much of the heat gained is released to the atmosphere using a fan that blows outdoor air over another heat-exchanging coil
  4. Pipework – liquid refrigerant moves back to the indoor room unit
  5. Indoor unit – the refrigerant is decompressed as is passes past the Expansion Valve, cools and can now extract heat from the room

In modern air conditioners this cycle can be easily reversed to provide energy-efficient indoor heating by extracting heat energy from the outside air. This heating (often called a heat pump) is effective even in freezing external temperatures.