These are the two ways an espresso machine heats water, and they trade off in opposite directions.
Thermoblock
A metal block with a channel through it and a heating element attached. Water is heated as it flows.
Fast — ready in 2 to 5 minutes, because there is very little mass to heat. Compact and cheap. No standing hot water, so no scale sitting in a reservoir.
Less stable. Temperature depends on flow rate, and it varies through the shot. Steam power is weaker and often intermittent.
Found in most entry-level machines and many bean-to-cup machines.
Boiler
A vessel of water held at temperature.
Stable — a large thermal mass resists change during a shot. Powerful steam, especially in dual-boiler and heat-exchanger designs.
Slow — 20 to 45 minutes to full thermal equilibrium. Heavier, more expensive, and scale accumulates inside.
Thermocoil
A middle option — a coiled tube in a heated block, with more mass than a thermoblock and less than a boiler. Sage/Breville use this widely, with a PID for control.
Genuinely decent, and a reasonable compromise.
Which to choose
Thermoblock if you make one or two drinks and want them quickly, or space is tight.
Boiler if you want consistent temperature and real steam power, and you can leave the machine on.
Thermocoil with PID if you want most of both.
Last reviewed 8 August 2026