Thermoblock, Single, Heat Exchanger or Dual Boiler?
The single biggest difference between espresso machines, explained without the engineering degree.
Luca Moretti · November 5, 2026 · 8 min
Espresso needs water at around 90–96°C. Steaming milk needs steam, which means water well above boiling. How a machine solves that problem — one heater or two, a boiler or no boiler at all — shapes almost everything about how it behaves on your counter.
There are four main designs. None is “best”; each suits a different kind of coffee drinker.
Thermoblock and thermocoil
Instead of holding a tank of hot water, these machines flash-heat water as it passes through a heated metal block or coil. They warm up in seconds, use little energy and are found in most compact machines.
The trade-off is stability and steam power. They are excellent for one or two drinks at a time, less so for a run of cappuccinos for guests.
Single boiler
One boiler, which switches between brewing temperature and steaming temperature. You brew your shot, flip a switch, wait for the boiler to heat up, and then steam.
Single boilers are simple, durable and often brew superb espresso. They suit people who mostly drink espresso, or who don’t mind a short wait before steaming milk.
Heat exchanger
A single large boiler held at steam temperature, with a tube running through it that heats fresh brewing water on its way to the group. You can steam and brew at the same time.
Classic heat-exchangers can run hot, and owners learn a “cooling flush” to bring the brew temperature down. Newer designs manage this automatically.
Dual boiler
Two separate boilers: one for brewing, precisely controlled, and one for steam. You get stable brew temperature and steam on demand.
This is the most capable design, and the largest and most expensive. For people who make several milk drinks a day, or who want total repeatability, it is worth it.
How to choose
Start with what you drink and how many.
Mostly straight espresso, one or two a day: single boiler or a lever.
A milk drink or two each morning: thermoblock or heat exchanger.
Several milk drinks, often for guests: heat exchanger or dual boiler.
You want to control every variable: dual boiler with PID or flow control.
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