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What is a thermoblock?

A thermoblock in an espresso machine is a feature in some espresso machines that flash heats water as it’s flowing through it. It is also sometimes called a thermocoil. It can be used for heating brew water or steam water. It’s essentially a heated metal coil through which water is pumped through, as the water flows through the coil it gets hotter and hotter until it reaches the grouphead or the steam wand of the espresso machine. By the time it reaches either one, the water temperature should be hot enough to brew an espresso shot or steam milk. Most super automatic espresso machines have thermoblocks as they provide faster warm up times than traditional machines do.

The pros of a thermoblock are that they warm up faster than traditional boiler machines and they usually cost less to produce then traditional boilers.
The biggest con is, generally, they don’t have good temperature stability. This is a drawback because temperature during extraction is one of the most important factors that effect the quality of flavor. If there is a lot of fluctuation of temperature during extraction, excess heat can cause bitterness and too little heat can cause sourness. Another con for thermoblocks is that they don’t tend to last as long as traditional boilers and they are more susceptible to limescale build up and failures related to that.