- Single-boiler espresso machines heat the brewing water via a heat exchanger in the steam boiler. The brewing temperature is then moderated by the thermosyphon, which cools the water enough for brewing and then returns the cooled water to the heat exchanger.
- The use of a heat exchanger means that single-boiler machines allow only indirect control over the group temperature. The temperature can also be affected by how much the group is used and by how often it is flushed between uses.
- Modern multi-boiler machines can offer very precise control over the group temperature. In some such machines, the group temperature may be offset slightly from the boiler temperature due to heat loss at the group. In this case, the group temperature must be measured by a technician using a Scace to ensure that the offset is accurate.
- Some machines use a thermostat or pressurestat to control the boiler temperature. This cycles the heating element on and off, resulting in fluctuating boiler temperature. By comparison, a PID controller varies the amount of power sent to the element, which means the boiler can be stabilised at a precise temperature.
- The effective brewing temperature in the puck depends on the water temperature but also on other factors, such as the flow rate and the temperature of the grounds.
- Changing temperature within the typical brewing range (90–96° C or 194–205° F) has a fairly small effect on extraction but a big effect on flavour. Increasing the temperature increases the extraction of nonpolar aroma molecules, relative to polar compounds. This means that changing temperature changes the proportion of the different compounds extracted.
Exit temperature The temperature of brewing water leaving the group in an espresso machine.
Thermosyphon The mechanism that controls the brewing temperature in most single-boiler machines. Water is superheated in the heat exchanger and loses heat as it passes to the group.