How does an espresso machine work: boilers, thermoblocks and pumps

The basic process

Water from the tank goes to a pump, then through a heater, and on to the group head, where the portafilter locks in. The group head spreads the water over the coffee through a shower screen. Pressure builds as the water meets the tightly packed puck, and espresso runs out of the portafilter spouts into the cup.

Two numbers matter most: temperature and pressure. Brew water is usually somewhere around 90 to 96 °C, and the standard pressure at the coffee is about 9 bar, roughly nine times the air pressure around us.

Pumps and pressure in bar

Most home machines use a vibration pump, a compact electromagnetic pump that is affordable and reliable. Higher end machines may use a rotary pump, which is quieter and can be plumbed into the mains. Some machines use a lever instead of a pump.

You will often see 15 bar or 19 bar on the box. That is the most the pump can produce, not the brewing pressure. In practice many machines limit pressure at the coffee to around 9 bar with a valve, and pressure also depends on the grind and dose. A higher pump rating does not mean better espresso.

Four heating systems compared

Milk needs steam, which needs water well above boiling point, while espresso needs water below it. How a machine handles those two temperatures is the main design choice.

What each means in the cup

For espresso and americanos, a single boiler or thermoblock machine is perfectly capable. The Gaggia Classic, for example, is a single boiler machine, while many Sage machines use thermocoil or thermojet heating, which warms up quickly. With these you brew first, then switch to steam and wait for it to heat.

If you make two or three milk drinks back to back, a heat exchanger or dual boiler saves time because steam is always ready. The Sage Dual Boiler is an example of the latter. These machines are bigger and usually need longer to warm up from cold.

For more on choosing, see our best espresso machines for beginners and Sage espresso machines compared. If you want the milk done for you, the best espresso machines with automatic milk are worth a look.

Other parts that matter

The portafilter holds the basket of coffee. Common sizes are 51, 54 and 58 mm, which decides which tampers and baskets fit. See portafilter sizes by machine. A solenoid valve on many machines releases pressure into the drip tray after a shot, which leaves a drier puck. A pressure gauge, where fitted, shows how much resistance the puck gives. Some machines have PID temperature control, an electronic controller that holds brew temperature more steadily.

The grinder matters as much as the machine. Espresso needs a fine, even grind, so a capable burr grinder or a machine with one built in makes a big difference. Our best coffee grinders for espresso guide covers this, and espresso dose and ratio explains how much coffee to use.

Note:Boilers hold hot water and steam under pressure. Never open a boiler, valve or the machine case while it is on or hot.

Frequently asked questions

How many bar do I need for espresso? About 9 bar at the coffee is the usual standard. A 15 or 19 bar pump is common and fine, since the machine and the puck limit the real pressure.

Is a dual boiler espresso machine worth it? It is worth it if you often make several milk drinks in a row and want to brew and steam at once. For mostly espresso, a simpler machine does the job.

What is a thermoblock? A heater that warms water as it flows through, rather than storing it in a boiler. It heats quickly and saves space.

How does a bean-to-cup machine differ? It uses the same principles but automates grinding, tamping and brewing. See what is a bean-to-cup coffee machine.