Temperature Stability in Thermoblock Machines

Brew temperature in a thermoblock machine depends on how fast the water moves. Fast flow gives the water less time in the hot metal. Slow flow gives it more.

What you'll need

• Your manual, for the maker's own settings and warm-up steps.
• The same coffee and the same grind setting, if you're comparing one shot with another.

What sets the temperature

One guide says stability depends on how the tube is installed, its length, the flow rate of the water, and the heating power.¹

With well-tuned machines, the guide says, this can be relatively or even very precise in standard operation.¹

Where drift comes from

• Slow flow. If the water flow is slow, for example because of very finely ground coffee, the water heats above the target.¹
• Many shots in a row. The same guide says that pulling many espressos or long drinks in a row can cool the surroundings, because large amounts of cold water flow through. The water then no longer heats to a sufficient brewing temperature.¹
• Block size. In most smaller machines the block is small, so it has to be heated well above the target. A larger block sits closer to the target and can deliver the temperature more consistently across flow rates.¹

What one manual says about its light

In one manual, during brewing or steaming the green READY light is off. That shows the heating system is on, maintaining the proper temperature for brewing or steaming.²

How to read a drifting shot

Say one shot tastes different from the last, and you changed nothing. Think about flow first. A grind that has slowed the water gives the tube more time to heat it. A run of back-to-back drinks does the opposite.¹

Neither is a fault. Both are how a flow-through heater behaves. Still, a machine that seems to act far outside its manual should go to the maker.

What you can do

• Pre-warm as your manual says. One manual tells owners to pre-warm all the internal components and the cups.²
• Keep your grind and dose steady between shots, so the flow doesn't change on its own.
• Leave any adjustment of the heater to the maker's own settings. Don't open the case or alter the electrics.

One comparison

Wikipedia says that in principle, TB machines yield a more stable brew temperature at the expense of steaming performance and speed, while HX machines yield better steaming. Here TB means thermoblock and HX means heat exchanger. That's Wikipedia's general in-principle statement, not a promise for any single model.³

The path the water takes is on the pump-to-cup page. For the switch to steam, see steam switching.