A split air conditioner has an internal fan unit fitted in the room to be cooled and an external compressor unit fitted outside the building. Both are connected with two copper pipes, through which refrigerant is passed in a continual loop.

The refrigerant will change its form from a liquid to a gas in different parts of the system. As it reaches the internal fan unit it turns from a liquid into a gas. Whilst doing this its temperature drops rapidly - like when you spray an aerosol can for a long time and it gets cold. This drop in temperature makes parts of the internal fan unit very cold indeed and as air is blown across them, the air becomes chilled and reduces the temperature in the room.

When the refrigerant, now in a gas form, is pumped back around the system and reaches the external unit, a compressor forces the gas back into a liquid state by compressing it. This activity produces lots of heat that is dissipated by a big fan in the external compressor unit outside the building.

So by continually expanding and compressing the refrigerant gas in the air conditioning system the heat is collected from the room and transferred to the outside air.