How an Espresso Machine Works: What's Really Inside?

Side View of a Vibiemme with the Covers Off

The boiler holds water. It is kept a little more than half full, and the water is heated by an electrically driven heater coil that's inside the boiler. The second picture (below) shows a heater taken from an espresso machine. As you can see, it loops around several times so that there is lots of surface area on the coil that is in contact with the water. This coil is screwed into the boiler so that its coils are under water when the boiler is full.

On the left side of the heater, you can see the threaded housing cap that screws into the side of the boiler, and you can see two electrical contacts on the outside of the cap. An electrical heating element runs within the coil, and it's electrically insulated from the surface of the coil (hence from the water). A controller measures the temperature of the boiler, and uses this to turn the heater on and off so as to maintain a fixed water temperature within the machine.

The Heating Element Sits Inside the Boiler

The diagram below shows how the boiler is used to produce both hot water and steam inside your espresso machine. The diagram shows that the boiler is kept a little more than half full of water. The heater that we've already described is shown in the water.

The Boiler With Hot Water & Steam Outputs, and Depth Control

The depth of the water in the boiler is controlled by the electrode shown in the middle of this picture. Control circuitry in the espresso machine (not shown) simply tries to pass current through this electrode. If the water level in the machine is lower than the tip of the electrode, no current can flow. In that case, the controller opens a valve that allows water to flow from an external water source (e.g., the building's plumbing fixtures) into the boiler.

Note that since the boiler is pressurized, the water source (building plumbing) must be pressurized too. It must have enough pressure to push water into the boiler when the valve is opened. Typically, this is 20-30 psi. When the water level rises to touch the tip of the electrode, the control circuitry turns the water intake valve off. In this way, the water level in the tank will stay constant.

In the picture, on the left side we show a tube that extends well below the water surface, and on the right side, we show another tube that ends well above the water surface. Both tubes have external valves that are controlled by the barista at the front of the machine. The valve on the left provides hot water, and the valve on the right provides steam. We would like to know the pressure in the boiler for making steam, and the extraction pressure for espresso. These are both measured with pressure gauges, and the gauge is usually on the front panel of the espresso machine, like the gauge shown below.

Pressure Gauge for the Boiler and for Extraction

Note that this gauge registers both the pressure within the boiler (the top arrow), and the extraction pressure (the bottom arrow). Both arrows should be within their green regions. We will typically run the boiler pressure (the pressure of the steam) at 1.5 Bars (see the gauge). If the pressure is much less than this, we won't get good steam. And if it's much more than this, it's in a dangerous regime. There is a safety valve that will blow (and will need replacement) if the pressure gets much more than 1.8 Bars.

The lower reading on the gauge is the espresso extraction pressure. This will not register the pressure until we pull a shot, but the pressure should be 9-11 Bars when we do. When we are not pulling shots, this gauge will indicate the water line pressure (your water source).

We've explained the water flow for hot water and steam, but what about the water flow for extracting espresso? Water for espresso does not come from the boiler. It is fresh water that comes in fresh from your building's plumbing on your supply line. This is shown in the next figure, below.

How Espresso is Extracted Using the External Water Source

The water for brewing espresso makes a loop through the middle of the figure (the blue path, marked with arrows). When the espresso machine is not brewing espresso, the solenoid valve (in red) disconnects the input from the pump, and the pump is turned off. It's the pump that will create the pressure (9-11 Bars net pressure), but the pump actually needs to put out a little more than this, as we'll see.

Note that this loop exits the grouphead (usually an E61 - as we'll discuss shortly), passes through the solenoid valve (normally closed to the pump input), goes through the boiler, and back into the grouphead. Usually, there's a flow control valve near the grouphead. Its purpose is to adjust the flow rate so as to control the temperature of the water at the group. The figure below shows this more clearly.

Water Flow for Espresso, and Steady State Thermosiphoning

As should be clearer in this figure, the outside water (from the pump) flows through an isolated chamber that runs through the boiler. The outside water does not mix with the boiler's water. It's simply run through the boiler to heat it, and to bring it to the correct temperature for extraction. The flow control valve adjusts the flow rate so as to regulate the water temperature at the grouphead.

When we say "E61 Grouphead," we are referring to the grouphead design that was introduced by FAEMA in 1961in their famous E61 espresso machine, which changed espresso machine design forever. It's a classic, and we sell them. Ask us if you're interested. The machine's a beauty!

This E61 grouphead is a heavy metal grouphead (heavy because it retains heat), which has an elaborate set of channels running through it. It has a water input (for making espresso), and a water output. See the picture of the E61 grouphead below.

The E61 Grouphead

The water output is there so that hot water can be running through the grouphead constantly, to keep it hot so that your espresso shots will be consistent. When we are not pulling a shot, the water pump is off, and the solenoid valve inhibits input from it. But the water can - and does - continue to flow in the circuit shown. It flows continuously in the loop though the gouphead, the solenoid, the boiler, and back to the grouphead. Why? And how?

Why? The water is kept flowing so that the grouphead will stay at a constant brewing temperature. And how? The water keeps flowing because of thermosiphoning, which is a passive physical mechanism. If we could measure the temperature of the water at various points in this cycle, we'd see that the water is the hottest as it leaves the boiler (which is a heat source), it cools down very slightly by the time it reaches the grouphead, it loses more heat as it passes through the grouphead (which is exposed to the room temperature), and it's at its coolest when it enters the boiler again. These small temperature changes cause the water to siphon automatically - to flow continuously around this circuit, which keeps the grouphead hot. This was a major innovation in 1961, and has stayed with us since.

Now we refer back to the previous figure to complete the description of the flow in an espresso machine when you brew espresso. You load the portafilter with ground espresso, tamp it down, and lock it into the E61 grouphead. Then you push one of the buttons on the control panel (shown in the figure). This turns the pump on, and opens the solenoid valve to the pump. Note that it does not shut the flow off coming from the grouphead - this water continues to cycle.

The water from the pump goes first goes through an electronic heat exchange unit (not shown here) to preheat it, and the preheated water is then mixed with the cycling water within the solenoid valve. We have not shown the heat exchange unit in our figure. But here is a picture of one.

A Heat Exchange Unit to Preheat Water for Espresso

The heat exchanger is very good at heating small amounts of water - as used in espresso shots, but will not sustain the flow of hot water indefinitely. This is why the hot water spigot on the espresso machine uses water from the boiler (more on this below). Economical home espresso machines do not have boilers - they'd be heavy and too expensive. Home Machines use heat exchangers for espresso, AND for steam. This is why home espresso machines simply will not produce the same richness of froth as commercial machines.

If you want hot water for a drink (an Americano, or tea, or hot cocoa), don't take the water from the hot water spout. This is BOILER water. It tends to be old, stagnant, and highly acid (for water). It also will have lots of mineral content picked up from scalng over time. It will not make good drinks. This is good water to use for rinsing things: preheating cups, rinsing out the portafilter, hot water to put on a cleaning rag, etc., etc. Many people never drain their boiler. So the water, while potable (because the heat kills everything), will not taste very good.

And don't run the hot water spout at the same time that you're pulling a shot! I'm sure you'll remember that when the hot water is run, the electrode in the tank will immediately turn on the water input to replace that water with COLD water. While the new water will heat quickly, if you're pulling a shot while running the water spout, this will throw the temperature of the water channel off during the shot. You'll get a poor extraction. Feel free to run the steam though, The steam hardly uses any water.

You SHOULD drain your boiler periodically, because this water IS stagnant, and can pick up impurities (e.g., milk residue if the steam wand is not used correctly). We recommend draining the boiler and refilling it monthly.

We hope that this article has been helpful to you.

...written by your friends at The Coffee Brewers