Warm air from the room is sucked in through a grille at the base of the machine

The air flows over some chiller pipes through which a coolant fluid is circulating. This part of the machine works just like the chiller cabinet in a refrigerator. It cools down the incoming air and a dehumidifier removes any excess moisture.

The air then flows over a heating element (similar to the one in a fan heater). On a cold day, this part of the unit may be turned right up so the HVAC works as a heater.

A fan at the top blasts the air back through another grille into the room. If the heating element is turned down, the air re-entering the room is much cooler, so the room gradually cools down.

Meanwhile, coolant (a volatile liquid that evaporates easily) flows through the chiller pipes. As it does so, it picks up heat from the air blowing past the pipes and evaporates, turning from a cool liquid into a hotter gas. It carries this heat from inside the room to the outside of the building, where it gives up its heat to the outside air. How? Just like in a refrigerator, the coolant flows through a compressor unit and some condensing pipes, which turn it back into a cool liquid ready to cycle round the loop again.

What happens to the heat? In the unit outside the building, there are lots of metal plates that dissipate the heat to the atmosphere. An electric fan blows air past them to accelerate the process.

Over time, the heat inside the building gradually pumps away into the outside air.