How a Thermoblock Heats Water as It Flows

The water is cold when it leaves the tank and warm when it reaches the coffee. The change happens during the short flow time, inside a tube you can't see.

What you'll need

• Your manual's description of the heating system.
• A rough picture of the parts: tank, pump, heater, brew head.

You don't need to open the machine to follow this. The case stays shut unless you're the maker or a qualified technician.

Step by step

1. The pump pushes the water. One maker's manual says the hot water is pumped through the filter into the cups. So the pump, not gravity, does the moving.¹
2. The water enters a tube. In the design described by one guide, a spiral-shaped tube runs through an aluminium block.²
3. A heating element warms the block. The element runs along the tube, so the metal around the water is already hot.²
4. The water takes the heat on the way through. There's no pause. It's heated while it flows.²
5. The water leaves at brewing temperature. That's the aim, at least. The next sections explain why it doesn't always land exactly.

Why the block runs hotter than the water

Water spends only a short time inside the tube. To hand over enough heat in that time, a small block has to be heated well above the temperature you want. One guide says the block must be heated significantly higher than the target temperature in most smaller machines.²

That has a side effect. If the water slows down, it stays in the hot tube longer and ends up hotter. The same guide gives a slow flow caused by very finely ground coffee as an example, where the water heats above the target.²

Tubes in tankless heaters

Household tankless heaters offer a hint. Wikipedia explains that they pack many small passageways, made from plates or tubes, into the heat exchanger. The result is lots of surface for heat to cross quickly.³

That passageway detail is about household tankless heaters. The sources saved here don't describe the size of a thermoblock's tube.

How the machine knows when to heat

You don't control the heater directly. The machine does. In the instruction text for one thermoblock machine, the green READY light turns on once the thermoblock has heated. During brewing or steaming that READY light is off. The manual says this shows the heating system is on.¹

Other models use other signals. Check which light, sound or screen message your own manual uses.

What this means for you

• A machine that has sat idle needs its heater to come up first. See why thermoblock machines warm up quickly.
• Flow speed and heat are linked. Temperature stability goes into what that does to a shot.