What Is a Thermoblock in an Espresso Machine?

A thermoblock is a flow-through heater. Cold water goes in, passes through a hot piece of metal, and leaves warm enough to use. Nothing sits waiting in a tank of hot water.¹

What you'll need

• Your machine's manual, open beside this page.
• The spec page or label for your model, if you want to know which heater it has.
• A little patience with the jargon. Each term gets a plain explanation the first time it appears.

Makers don't always use the word "thermoblock" on the box. The manual's description of the heating system is the place to look. It's also the only place that tells you what your own model does.

What's inside

One guide describes the design like this: instead of a heated water reservoir, a spiral-shaped tube is threaded through an aluminium block. A heating element runs along the tube.¹

The water is heated as it flows through the tube. That's all "flow-through heater" means.¹

The block is just metal that holds heat. The pump pushes water through the tube, and the metal hands its heat to the water on the way. The tank at the back of the machine still holds plain cold water. It isn't the heater.²

The same idea turns up outside coffee. Wikipedia describes tankless water heaters as water heaters that instantly heat water as it flows through the device. A thermoblock is that idea shrunk down for a cup.³

What it does when you switch on

One maker's manual for a thermoblock machine puts it simply. When the machine is on, the thermoblock heating system heats up. The same manual gives the first heat-up time as under one minute at normal room temperature.²

Your model may differ. Check the heat-up time and the light signals in your own manual.

Where it fits among machine types

Thermoblocks aren't only found in machines that use one heater. Wikipedia describes a "dual heater" layout with a boiler for brewing and a separate thermoblock for heating water to steaming temperature.⁴

So the word tells you about the heater, not about the whole machine. Two machines that both mention a thermoblock can still be built quite differently.

What to expect from the design

• Heat-up is short. That's the main advantage the sources name over boiler machines.¹
• Brew temperature depends on how the tube is fitted, how long it is, how fast the water flows and how much power the heater has.¹
• Steaming milk is generally harder on this kind of machine.¹

The next pages cover each of these. Thermoblock vs boiler compares the two heating styles. How a thermoblock heats water follows the water step by step. Temperature stability explains why a shot can drift.