The ‘exit temperature’ refers to the temperature at which brewing water leaves the group and hits the coffee. Different machines control this in different ways and, depending on the machine design, the temperature set point may be more or less easy to adjust. Modern multi-boiler machines typically allow precise control of the exit temperature.
It’s important to know how stable your machine temperature is and whether it will be affected by drawing hot water from the steam boiler or flushing the group excessively, for example. Conversely, some machines may need a longer flush before the brewing temperature stabilises.
In commercial single-boiler espresso machines, the brewing water is heated in a heat exchanger — a pipe that passes through the inside of the steam boiler. When you brew a shot, cold water enters the heat exchange. As it passes through on its way to the group head, it is rapidly heated by the large mass of hot water and/or steam surrounding the heat exchange.
The temperature inside the steam boiler — typically between 118–124° C (244–255° F) — is much higher than the temperature required for coffee brewing. Because of this, some sort of mechanism is required to remove the excess heat from the brewing water before it exits the group. The most common way to achieve this is by use of a thermosyphon.
Illustration: the thermosyphon in a typical single-boiler espresso machine.
The steps in the image above show how a thermosyphon works: 1) Water in the heat exchanger is heated to >100° C (>212° F) by the heat from the steam boiler. 2) The hot water passes into the top of the group head. 3) The group head loses heat to the atmosphere, cooling it (and the water in it) to approximately brewing temperature. The large thermal mass of the group head helps stabilise the temperature. 4) Cooler water sinks, and so it passes out of the base of the group and back to the bottom of the heat exchanger.