The heat exchanger inside your machine is contained inside the main boiler
and consist of a tube that runs through the boiler. As the water inside the boiler heats up to operating temperature, heat is then transferred through to the walls of the tube and into the water held inside the heat exchanger.
As the the water temperature inside the heat exchanger increases, it naturally rises towards the group head, in the same way hot air rises. The group head
is quite a heavy lump of brass, which causes this water to cool down as it circulates inside the group head.
Once the water temperature in the group head starts to fall, this cooler water then drops back down the return pipe and is returned into the bottom of the heat exchanger, ready for the process to repeat.