Boiler

The pressurised vessel inside an espresso machine that heats water to brewing or steam temperature. Boilers are made from copper, brass, or stainless steel, and machines use one, two, or three of them depending on design.

Why it matters — The boiler architecture defines how your machine behaves. Single boilers heat one vessel for both brew and steam, with a switch between modes. Heat-exchanger (HX) machines use a single steam boiler with a brew line running through it. Dual-boiler machines have a separate boiler for brewing and one for steaming — the most stable and flexible setup.

Good to know

• Copper boilers heat fast and conduct beautifully, but require descaling discipline in hard water areas.
• Stainless steel boilers resist corrosion and scaling — common in modern prosumer and commercial machines.