Multi-boiler machines allow the brewing water temperature to be set independently from the steam boiler. This allows more precise control of the brewing temperature and makes it much easier to change the temperature. The temperature can usually be changed in the machine’s programming menu.
In many multi-boiler machines, however, the brewing temperature is measured inside the boiler, even though the brewing water loses heat to the atmosphere on its way to the group. The boiler temperature usually includes an offset to account for this, setting the boiler temperature 1 or 2 degrees higher than the desired brewing temperature. This means that the actual brewing temperature may not be exactly the same as what it’s set to.
When multiple groups are supplied from one brew boiler, they may also have slightly different temperatures from each other, depending on the distance from the heating elements.
To overcome these issues, many modern machines incorporate a second heating element embedded in the group head itself to bring it up to the desired brewing temperature. As the water passes through the group, it is heated or cooled to the precise temperature desired by the large thermal mass of the group. Once such machines are correctly calibrated, the brewing temperature can be controlled very precisely, to within as little as one-tenth of a degree.