A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including boiler-based power generation, cooking, and sanitation.
With regard to coffee, a boiler will normally be discussed in relation to an espresso machine. All espresso machines have a boiler for heating the water. Most commercial espresso machines will have a double boiler system; one boiler for heating the coffee and one for heating steam. A steam based, domestic espresso machine. on the other hand, will have a single boiler which is responsible for heating the water for coffee extraction and the water for milk steaming.
Coffee boilers are generally made out of a metal that is a good heat conductor.The most common metals are Aluminium, brass, copper and stainless steel. Aluminium and brass are often used in cheaper espresso machines while copper and stainless steel are found in more expensive, commercial coffee makers.
A single boiler espresso machine will typically not be powerful enough to produce high quality micro foam. Therefore, it is difficult to achieve latte art for milk based espresso machines such as cappuccino or latte.