Single Boiler Espresso Machines

In single boiler, or heat exchanger, espresso machines there is an additional tube within the boiler. When the barista pulls a shot, the water flows from the water source through this tube located in the boiler. As it flows through this tube, it reaches the appropriate temperature for brewing (around 200 degrees give or take) and flows into the group head for shot extraction. Any leftover water in this tube after the shot will most likely need to be flushed as it will keep getting hotter because of the temperature of the steam water.