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How an Espresso Machine Works and What You Should Look For

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By: Will Libby

Last Updated:

Rancilio Silvia internals
The main parts inside my Rancilio Silvia

Open up an espresso machine, and you’ll see that the inside looks more like a supercomputer than a piece of coffee equipment. How does this rat’s nest of wiring and metal pipes produce the drink we all know and love?

The principles of how an espresso machine works are broken down into four parts: a water reservoir, heating element, pump, and brew head. Each of these is simple to understand, and how they work together makes a lot of sense once you understand each component.

I’ll break down every piece of an espresso machine in detail and offer a few things to look for when trying to find the perfect machine for your needs.

Understanding Espresso

“Espresso” refers to a specific method of brewing coffee. In this method, hot water is forced under pressure through a small bed of tightly compacted coffee grounds. This often takes less than 30 seconds, and the result is a very small amount of highly concentrated coffee. A standard double-shot of espresso is only about two ounces but contains as much caffeine as a 12-ounce cup of filter-brewed coffee!

Brewing espresso requires three things: water, heat, and pressure. The job of an espresso machine is to bring these three elements together to consistently extract all the rich flavor contained in finely ground coffee. To do this, an espresso machine requires four mechanical components:

  • Water source
  • Pump
  • Heating System
  • Brew Head
  • Steam Wand (optional)
infographic of how an espresso machine works, showing flow of water through an espresso machine

Water Source

Something all coffee drinks share in common is water. 98% of coffee is water, or in the case of espresso, more like 92%.

There are two primary systems that espresso machines use to draw water for brewing: reservoirs and plumbed-in water.

A reservoir is common in most home espresso machines. These are small holding tanks inside the machine where water is stored before being heated for brewing. These vary in size depending on the machine, but they share one thing in common. They need constant refilling to ensure there’s sufficient water for the machine to use.

A plumbed-in system, on the other hand, draws water directly from an external water source. This can be a larger water tank or a municipal water source. Some home machines have the option to use either a reservoir or plumbing, but plumbing is more common on commercial machines. 

Pump:Generating 9 Bars of Pressure

Pressure is the key ingredient that separates espresso machines from regular coffee makers. Drip coffee and pour-overs use gravity to let water “fall” through the bed of coffee. Espresso machines force the water through the coffee grounds using pressure.

The ideal pressure for brewing espresso is 9 bars. A “bar” is equivalent to normal air pressure at sea level. Put in another way, 9 bars is the same as about 130 psi. For every square inch of the coffee bed, an espresso machine should exert 130 lbs. of force!

To ensure a consistent 9 bars of pressure, most espresso machines generate more than 9 bars and use an “Over Pressure Valve” (OPV) to moderate the pressure.

For example, an espresso machine might create 15 bars of pressure at the pump, and then OPV will meter that down to 9 bars before forcing the water onto the bed of coffee. Starting with more pressure than needed and then reducing it at the brew head ensures you never drop below the required pressure for even extraction.

bars of pressure for espresso gauge at 9-10
Bars of pressure for espresso gauge at 9-10

Espresso machines typically use either a vibratory pump or a rotary pump to generate the necessary pressure. Vibratory pumps use an electromagnet to drive a piston that pushes water through the machine, and they’re more common in home espresso machines.

A rotary pump is more expensive since it uses a small, but powerful, electric motor, and these pumps are more common on commercial machines.

Some machines use steam to produce pressure, but I don’t recommend them if you want any consistency in your espresso. These are great for camping or for having a cheap, portable espresso option. However, they’re not an ideal choice for a home machine you’ll use regularly.

Boilers and Heaters

You also need heat, or energy, to extract the rich flavors from your espresso. Espresso machines all use a heating system to get water up to the ideal brewing temperature of 200 degrees F (90 C). What type of heating system is important, since good espresso is all about consistent water temperature. The main heating systems are: thermoblocks, boiler, and heat exchanger.

espresso machine heating systems: thermocoil, boiler, and heat exchanger

Two common heating systems used in home espresso machines are thermoblocks, or the more modern counterpart, the termocoil. The principle of these two systems is the same.

Water flows through a metal channel on its way from the water reservoir to the brew head. The metal piping that the water flows through is being heated via a metal heating element (usually stainless steel). As the water flows through the channel, it absorbs heat from the metal. By the time it reaches the brew head, it’s been heated up to the chosen temperature.

Thermocoils and thermoblocks excel at price and speed of start, but the temperature is unstable given the thin piping. These are common on entry-level espresso machines.

The next type of heating element is a boiler. A boiler is like a water reservoir and heater in one. It’s essentially a pot of water on the stovetop, and turning the espresso machine on turns on the stove burners. In a boiler, water is heated before it flows to the group head or steam wand, whereas thermocoils and thermoblocks heat water as it flows to the group head. 

Espresso machines can be either single-boiler or dual-boiler machines. A single-boiler machine uses just one heating element for both brewing coffee and steaming milk. A dual-boiler machine uses one heating element for brewing, and one element for steaming.

Some single-boiler machines utilize a heat exchanger to allow simultaneous brewing and steaming. This system uses a boiler heated to boiling temperature with a metal pipe inside the boiler. The boiled water is used for steam, and the pipe inside the boiler draws cool water from a reservoir that’s heated along its way to the brew head.

A heat exchanger is more affordable than a dual-boiler machine, since it still only uses one heating element, but it’s less precise.

One final note on espresso machine heating systems: consistency is key. It’s one thing to have a powerful boiler, and it’s another thing entirely to have a consistent boiler.

Getting the most flavor out of your coffee requires having the water be the same temperature every time you pull a shot. To that end, some espresso machines are equipped with a PID system. This is a fine-tuned control system that lets you program the exact temperature of your machine. It can hold a boiler, thermocoil, or thermoblock at your chosen temperature for consistent extraction in every shot.

Brew Head / Group Head

The brew head is where the magic happens. This is where water leaves the espresso machine and flows through the coffee bed, brewing espresso.

If you look at the brew head (also called a group head), you’ll notice a mesh screen covering it. This screen helps distribute water evenly over the entire bed of coffee so that all the grounds are extracted evenly.

Your home espresso machine will only have one brew head, but commercial machines can have two, three, or even more group heads for brewing multiple shots at once. 

Brew heads should be heated to ensure temperature consistency between shots. If not, it can cool the water before it contacts the coffee, affecting extraction.

The most common way to heat a brew head is by connecting it to the boiler so that it’s always in contact with heated water. This is called a saturated group head. Group heads that aren’t saturated can still be heated separately using an electric heating element, but this is far less common. Usually, I recommend running water through the brew head to pre heat. This “blank shot” will get your brew head consistent with the boiler and it cleans the shower screen.

Portafilters

Working in conjunction with the brew head is the portafilter, short for “portable filter.” It holds the coffee grounds underneath the group head.

Coffee is ground, placed in the portafilter, tamped, and then locked into the brew head. The bottom of the portafilter is a metal bowl with tiny holes cut into it. This lets liquid espresso flow through while trapping the coffee grounds.

Portafilters come in two different styles: spouted and bottomless. Spouted portafilters are cleaner but cover the bottom of the portafilter and don’t let you view the extraction process. Bottomless portafilters leave the perforated metal uncovered. You can watch extraction happen, but it can also get messy if your espresso starts channeling.

portafilter types spouted and bottomless
Two portafilters: bottomless/naked (top) and spouted (bottom)

Portafilters also come in different sizes, and their size matches the size of the group head of their paired espresso machine. portafilters are measured by diameter, and the standard commercial portafilter size is 58mm.

However, a lot of home espresso machines are smaller at 54mm (Breville) or 51mm (DeLonghi). I think size impacts espresso to an extent, where wider is better. You can’t swap out portafilters of different sizes since it’s based on the group head.

Steam Wand

Breville Barista Pro milk steaming
Traditional steam wand on a Breville Barista Pro: more versatile and better for latte microfoam

The steam wand is an optional (but important) accessory that many home machines use. It’s a thin metal tube connected to a boiler, thermocoil, or thermoblock. The water is heated to a temperature hot enough to produce steam, and you can open a valve to let that steam out of the ends of the steam wand under pressure.

The steam wand aerates milk to make lattes, cappuccinos, and the like. Some machines have automatic steam wands that do this work for you, but many machines use a traditional, manual steam wand.

If you know how to steam milk, a manual steam wand gives you total control over the milk’s texture and temperature.

If you’re using a single-boiler machine, you can’t use the steam wand while brewing. Since steaming requires boiling water, and brewing uses water that’s about 205o F, you have to give the water time to reach a different temperature for each function.

A dual-boiler or heat exchange machine solves this problem by allowing you to have water set for two different temperatures at the same time.




What To Look For When Choosing An Espresso Machine

Now that I’ve gone through every component, let’s review what’s important when choosing an espresso machine.

Does It Generate 9 Bars Of Pressure?

Without pressure, an espresso machine is just an expensive drip coffee maker. A good quality home machine will generate at least 9 bars of pressure. To guarantee consistency, you can get a machine that generates more than 9 bars and uses an OPV to moderate pressure.

So long as your chosen machine uses a pump and not steam pressure, you should be able to produce enough pressure to extract sufficient flavor from your coffee.

What Is The Temperature Control?

How much control you have over water temperature is largely a matter of preference and budget. A thermoblock with no PID will be the cheapest heating option. But, it gives you almost no ability to fine-tune water temperature. By contrast, a dual-boiler PID machine will be expensive but gives total control over brewing temperature.

Next, consider how much control you want and how much inconsistency you can tolerate in your water temperature. A heat exchange will be less consistent and precise but also more affordable. A dual-boiler system with a thermoblock or thermocoil will give you more control. A dual-boiler that uses an actual boiler provides the most temperature consistency. Combine that with a PID, and all of a sudden, water temperature is a consistent guarantee, not a brewing variable.

Also, ask whether or not you want to be able to brew coffee and steam milk at the same time. If you do, then you have to get either a dual-boiler or a heat exchange machine. Personally, I don’t think this option is necessary for any home barista. I’d recommend a dual dual boiler for advanced temperature stability, not the 30 seconds saved by simultaneous brew and steam.

What Kind of Steam Wand is Included?

First of all, do you even want a steam wand? If you only drink espresso or Americanos, you can get a far more budget espresso machine.

If you do want to steam milk, decide if manual or automatic is better. If you have experience, I recommend manual since these tend to have better steam pressure and give you complete control. An automatic steam wand is also a great option if you’re new to espresso and don’t have prior experience steaming milk. The Breville Barista Touch or Philips LatteGo are good automatic milk steaming options.

Whichever you choose, make sure the machine can generate enough steam pressure to aerate your milk. A dual-boiler (even dual thermoblocks, like the DeLonghi) do this best, but a single-boiler machine also works.

A heat exchange is functional, but I’ve found that it’s usually less powerful than having a dedicated steam boiler.

What’s the Build Quality?

An espresso machine is a big investment, so you want to make sure it will last. Many machines are built of stainless steel, and they’re designed to be sturdy and durable. Some manufacturers use plastic on the exterior to save on costs, but it doesn’t mean these machines are cheaply made.

Reputable manufacturers like Breville and Gaggia produce affordable home machines with plastic exteriors that are still fairly durable.

Exploring Your Options

Now that you know how an espresso machine works, it’s time to find your perfect machine! Explore our favorite espresso machines and learn how to maintain an espresso machine to get started on your home barista journey.

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Will Libby
Will has done everything in coffee from cafe manager, educator, and roaster, and he owns Color Wheel Coffee Roasters with his wife, Brianna. His coffee journey began in college, when he got his first barista job at a local coffee shop. He was fascinated by the care and attention that went into brewing each cup, and he tried to learn everything about coffee, from seed to cup. Now he's taken on writing to educate others about specialty coffee. His favorite way to brew coffee is in a Chemex. When he's not roasting or writing about coffee, he can often be found writing music or trying to learn a new language.
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