Finally, in some espresso machines for commercial or home use, water for brewing is heated in a separate chamber, which requires two separate boilers. This is found primarily in higher-end machines, though it is also found in some mid-range machines, overlapping with HX.
The term "Dual Boiler" is used narrowly for machines with two separate boilers, and more broadly for what are more properly called "Dual Heater" (DH) machines featuring a boiler for brewing and a separate thermoblock (TB) for heating brew water to steaming temperature – opposite to HX machines, where the boiler is at steaming temperature and is cooled to brewing temperature.
In principle, TB machines yield stabler brew temperatures at the expense of steaming performance and speed, while HX machines yield better steaming at the expense of stable brew temperature. True DB machines provide stable brew temperatures and fast steaming, but are larger and more expensive.