Stainless steel boilers, PID, thermoblock, thermojet, brew head, heat exchange… Is your head spinning yet? The inside of an espresso machine is a tangle of wires, metal or plastic, and steam.
An espresso machine is made of many different components that work together to push water through your puck so you have a delicious shot of espresso. But, understanding all espresso machine parts, where they are located, and what exactly each component does can be challenging.
To make it easier to understand how your espresso machine produces your caffeine, I’ll break down exactly how espresso machines work. I’ll talk about everything from the water reservoir, boiler, brew head, steam wand, and much more. Here’s your one-stop guide on how an espresso machine works.
Breaking Down How an Espresso Machine Works
To understand how espresso machines work, you should know their parts and functions. Each machine component has an important part in brewing your favorite cup of coffee.
You force water through finely-ground tamped coffee beans when making espresso. The water temperature and pressure at which the water goes through the coffee are extremely important to get a flavorful espresso cup, which is why you should know how the espresso machine does this.
To sum it up, there are four phases to making espresso:
- Water source (reservoir)
- Group head and steam wand
These are all the stages the machine goes through to pull your espresso shot. Here’s an explanation of each of these in detail, according to different kinds of espresso machines.
An espresso machine can’t work without water. Home espresso machines get water through one of two sources: a reservoir located in the machine (a more common option) or a plumbed connection to a water supply. Which water source the machine will have depends on how it’ll be used.
Machines that brew a few espresso and lattes a day usually have a reservoir, while commercial machines that make hundreds of drinks a day are more commonly connected to a water supply.
Some hybrid espresso machines have both options. This is a good choice if you want flexibility and the option to plumb-in in the future.
A water reservoir is a good option because you can install a water filter and don’t have to only use what comes out of the tap. The downside is that you have to refill it.
Also, the water reservoir is almost always removable. It holds cold water and has a lid that keeps the water safe from dust and debris. However, the water reservoir doesn’t have pressure to force water through condensed coffee grounds, which is where the pump comes in.
The pump is the heart of your espresso machine. The water needs enough pressure to push through a tightly packed bed of ground coffee — 9 bars of pressure to be exact. The pressure should be between 9 and 15 bars to get good espresso. A typical espresso machine produces 9 bars of pressure, but some high-end machines can produce 15.
Nine bars is nine times the atmospheric pressure at sea level, or 130 PSI. Here’s an example to give you an idea of how strong this pressure is: car tires need between 32 and 35 PSI, and espresso machines need 130 PSI.
The pump draws water out of the reservoir and pumps it into the heating chamber (more on heating later) and through the coffee grounds. The pressure needs to be strong enough to extract the most flavor, oils, and caffeine from ground coffee.
That’s why it’s important to tamp the coffee — the tamp sustains the pressure and doesn’t allow the water to go through the coffee too quickly.
Original espresso machines used pistons attached to levers, and baristas had to pull a lever to make the water go through the coffee. Modern machines don’t need this much manual labor. Instead, they use two kinds of electric pumps: vibratory and rotary.
A vibratory or a vibe pump is commonly used in home espresso machines. It’s small and has an electromagnetic workhorse — a magnet with a piston inside a metal coil. Electrical current goes through the coil, which causes the magnet to move the piston back and forth, which pushes the water through the machine. It achieves 60 pushes per second on average.
Vibe pumps are more common in espresso machines. They only create pressure when you pull the shot. It’s inexpensive and easy to replace.
A rotary pump is commonly used in commercial espresso machines. These pumps create a constant supply of pressure, not only when you pull a shot.
A rotary pump is more complex compared to a vibratory one. It has a motor that spins a disc offset in a large round chamber. The disc has sections divided by veins.
As it spins, the veins press on the wall of the outer chamber, which diminishes the size of the section and creates pressure. Water goes in through the large phase and is pushed out as the section shrinks.
Rotary pumps are quieter, have more consistent pressure, and last longer compared to vibratory. However, both kinds of pumps can pull good espresso.
This technically isn’t a pump, but it’s another way in which machines create pressure for coffee drinks.
These machines use steam and heat to force water over the grounds. The machine creates steam pressure by getting water to the rolling boil inside a sealed chamber. The chamber heats and transforms the water into steam. Then, the steam is forced over the coffee grounds.
Steam machines are cheaper compared to pump ones. They can generate three bars of pressure, which isn’t enough for a flavorful espresso shot. Also, the pressure they create is inconsistent, which results in weak or bitter espresso, and they can’t create espresso crema.
Here’s a good video comparing pump vs. steam-driven espresso machines:
Boiler (Heating System)
When the water is moved forward by the pump, it’s time for the heat. To pull a good quality espresso, you need water at optimal temperature, which is where the boiler comes in.
The water goes into the boiler through a one-way valve where it’s heated. The boiler needs to heat and hold the pressurized water that comes in from the pump.
Newer espresso machines use electric heating elements, which means when they are on, the electricity goes through the element and generates heat, and when they are off, they are inert.
The size of the boiler is an important consideration. A bigger boiler means more drinks, but also more energy spent and more wait time until the water is heated.
Espresso machines can have different kinds of boilers. I’ll cover each one, or you can check out this video for a good rundown of heating systems.
Thermoblock is a metal block made of aluminum and stainless steel with heating elements and a pipe for water. Water goes along the pipe and picks up heat from the block, so it exits the thermoblock at the desired temperature.
Thermoblock used to have a reputation for temperature instability and were common in entry-level machines. However, a lot of that changed in recent years due to technological advancements.
Newer espresso machines use thermoblocks with PID (more on PID below), so you can set your temperature and have excellent stability.
Newer thermoblock machines have aluminum outside and stainless steel inside, which is very resistant and doesn’t scale.
You can’t steam and brew simultaneously with a thermoblock unless the machine has two thermoblocks. That being said, new machines, such as the Breville Barista Pro, heat up in seconds, so you don’t have to wait long.
Single boilers use a thermostat with a top limit, so when the water in the boiler reaches that temperature, the heating element turns off and back on once the temperature cools down.
As the name says, a single boiler is one boiler. Water for both brewing and steaming milk comes from one tank. This means you can’t brew and steam simultaneously because brewing and steaming temperatures are very different.
You’ll have to wait until the water heats up or cools down enough to do each action. If the wait time is long, your espresso can cool down while you wait for the steam to kick in to steam milk for a latte.
A single boiler is most commonly used in a home espresso machine because of its small size. It can have two thermostats — one that heats water for brewing and another that heats water for steaming. You need a thermostat that can heat 12 to 16 ounces of water to get the correct temperature for espresso.
Heat exchange boiler is usually used in bigger homes or commercial machines. This is one large boiler with a pipe inside. It’s filled about 3/4 with water brought to a boil and controlled via pressure snap or PID. As this water is boiling, it’s used for steaming milk.
To brew, the heat exchanger pulls water from a cold reservoir that goes through the heat exchanger and comes out at the top.
The heat exchanger does this by continuously pushing water through the separate element, into the group head, and back down. This means there’s no wait time between brewing and steaming, as is the case with a thermoblock and single boilers.
Heat exchanger machines have a very stable temperature.
As the name says, a dual boiler espresso machine is a machine with two boilers. It has two separate tanks instead of one. One tank is for brewing espresso, while the other is for steaming, which means you can do these actions simultaneously. There’s no need to wait for the water temperature to change, as is the case with single boilers and thermoblocks.
Dual boilers also have stable temperatures, as each tank can hold water at the appropriate temperature for the action the machine needs to perform.
Dual boilers have the edge over heat exchangers because each boiler is independent. You can raise or lower the temperature without affecting the steam — you’ll lose steam power if you lower the heat on a heat exchanger.
Overall, a dual boiler is a great choice if you want a lot of control over coffee and steam if you drink milk-based beverages, and you don’t want to wait between brewing espresso and steaming milk.
Temperature Control: the Importance of a PID Controller
Brewing espresso is an art, and even a small change in water temperature can affect the shot quality. If your espresso shot is cold, it’ll be sour and under-extracted, and if it’s too hot, it’ll be bitter and over-extracted. That’s why manufacturers use tools that help temperature stability, such as the PID.
PID stands for a proportional integral derivative controller. It controls the heating element and allows you to change the temperature in single-degree increments, and holds that temperature stable.
To understand why you need a PID, remember that heating systems are electric and therefore are either fully on or off. The “on” position boils water if left unchecked and even when the heating element is switched off, it’s still hot and continues to heat water, that causes the temperature to overshoot the perfect water temperature right around 200O F.
The same thing happens when the boiler cools too – the water temperature will undershoot because it takes time for the heating element to get hot enough again to start heating water.
Essentially, the temperature fluctuates, so the thermostat system isn’t accurate. PID fixes this.
PID is connected to the heating element and the temperature probe in the boiler. It reads the input from the probe and cycles on and off the heating based on an algorithm.
Here’s a great video that explains what’s PID and how it works.
The group head is where the magic finally happens — this is where hot pressurized water reaches a tamped coffee puck. It has four parts:
- A portafilter
- A portafilter lock
- Pump activation
- A water pathway from the boiler to the portafilter.
An espresso machine can have 1 or 2 group heads, but you’ll probably only need one for home use.
Saturated Group Head
A saturated group head is exposed to the boiler, so it’s an extension of the boiler. It comes to the same temperature as the brewing water quickly, because, as the name says, it’s flooded (saturated) with hot water. It’s also stable in terms of temperature because it draws water from directly inside the boiler.
This group is welded straight onto the boiler. The hottest water rises to the high point at the group head, which radiates heat into the air outside. The cooler water siphons down to the boiler, where it gets reheated.
Saturated group heads are more expensive to manufacture. It requires precise welding and a thick metal build. It’s also more difficult to repair.
Semi-Saturated Group Head
A semi-saturated group head is separate from the boiler via a heat exchanger. The water intake nozzle goes from the extended boiler into the dry area above the dispersion. From there, it goes to a three-way valve that’s connected to the dispersion and third waste line.
These group heads are cheaper to make and easier to repair, but the temperature is less stable compared to saturated ones.
Portafilter is short for portable filter. It is a filter basket with a handle. The basket has a small removable screen where the coffee is packed, and the espresso comes out at the bottom.
You place the coffee grounds into the portafilter, compress them by tamping and insert and lock the portafilter into the espresso machine. This is the process for manual espresso machines. If you have an automatic machine, the machine will do the work for you.
Portafilters come in different sizes, but 53mm and 58mm are the most commonly used, with 58mm found more in commercial than home espresso machines. Both of these can produce good espresso. The difference is if you’ll need a 53mm or a 58mm tamper, depending on the size of your portafilter.
There are two types of portafilters: spouted or bottomless (aka naked). Each has pros and cons.
A bottomless, or naked, portafilter has an exposed basket, which can help you dial in the grind more accurately. The bottom of the filter is open, and with no basket, it looks like a metal ring with a handle.
When you pull the shot, the water goes through the puck and creates a column of liquid that flows from the basket into the cup.
The bottomless portafilter is a great learning tool. You’ll have a clear view of how the water and ground coffee transform into an espresso drink. The extraction process is exposed, so you can make adjustments and have less guesswork. It’s also easier to fix issues such as channeling because you’ll be able to see them.
Spouted portafilters come with one or two spouts, so you can pull shots into two separate cups if you want. If you want to split shots, you’ll have to use a spouted portafilter.
Spouted portafilters are easier to use. They pour cleaner – right from the spouts – and espresso doesn’t spray out the side on a spouted portafilter, which happens a lot with naked portafilters.
However, you can’t see the shot develop with the spouts, so you won’t be able to diagnose channeling issues. the spouts also reduce clearance for the cups, so it’s more difficult to fit larger cups underneath.
A steam wand is a thin metal nozzle usually placed at the side of the espresso machine. It heats and froths milk for espresso-based drinks such as lattes and cappuccinos. An espresso machine needs steam to create steamed milk, for which water needs to be boiled.
The wand is attached to a heating vessel, and when you place the valve in the steam position, the steam comes out from the heating vessel out of the wand and into your milk.
Brewing and steaming temperatures are different — 200℉ for brewing and 212℉ for steaming. Different machines solve this in different ways. Single boiler machines can have two thermostats — one has a temperature range for brewing and the other for steaming.
But you can’t steam and brew at the same time. You can steam and brew at the same time with heat exchange machines and dual boilers.
Traditional Steam Wand
A traditional wand shoots jets of steam into the milk through small holes on the wand’s tip. This wand gives you a precise way of steaming. You can create smooth and creamy milk or thin microfoam needed for latte and latte art.
You can adjust the steam intensity on some machines via a dial. You’ll need a milk pitcher in which you pour milk, then insert the wand into the milk container by holding the cup up to the end of the wand.
Pannarello Steam Wand
A Pannarello wand is attached to the steam arm of the espresso machine. It can be made of plastic, metal, or a combination of both. It’s popular among first-time wand users as it’s easy to use.
Where traditional wands have several holes, Pannarello has one small air intake hole on the side. The air is sucked in through the hole and injected into the milk, which creates froth.
Pannarello’s don’t have a lot of steaming power, so they try to infuse more air into the milk to maximize the frothing. They can create thick foam for cappuccinos but not really thin microfoam needed for latte art.
Pannarello’s are usually found on entry-level machines, and several Gaggia machines are equipped with them.
You should purge Pannarello before use to remove water that may have collected inside. Once this is done, place the tip of the Pannarello inside your milk pitcher. The tip should be just beneath the milk surface.
What to Look for When Choosing an Espresso Machine
I talked about the most important parts of an espresso machine — everything from the start at the water reservoir to the end at the group head and portafilter. Here’s a quick recap of how espresso machines work.
Does it Generate 9 Bars of Pressure?
Your espresso machine brewing pressure should be 9 bars for pulling good espresso shots. Everything under this, and you’ll have weak and under-extracted espresso. If the pressure exceeds 15 bars, your espresso will be over-extracted and bitter.
Also, choose a pump espresso machine over a steam-driven machine, as they are more efficient and can achieve sufficient pressure to yield good espresso.
Heating Element and Temperature Consistency
When it comes to heating elements, you can choose between thermoblocks, heat exchange, single and dual boilers.
Thermoblocks have improved in recent years, especially when used with a PID. However, you can’t steam and brew simultaneously.
The heat exchange system allows you to brew and steam simultaneously. It’s one large boiler that continuously pushes water through a separate element, so there’s no wait time. It’s usually found in commercial espresso machines.
A single boiler also can’t steam and brew at the same time. Most modern espresso machines have two thermostats, so there’s a short wait time.
Dual boiler machines have two boilers. These machines have stable temperatures as water comes from separate tanks for each action. They are great because you can brew and steam at the same time, but also come at a premium.
Overall, you should get one with PID controller no matter which of these machines you go for. PID will keep your temperature stable, and you won’t end up with weak or bitter espresso.
High-Pressure Steam Wand
If you like to drink milk-based drinks, a milk frother is an important consideration. You can go for an espresso machine with a traditional wand. These have several air holes and are usually able to create foam for cappuccinos and lattes.
Or, you can go for a machine with a Pannarello wand. Pannarello is easier to use. It’s usually found on entry-level machines, and it struggles to achieve enough pressure. It has only one air intake hole and usually can’t create the thin microfoam needed for lattes.
Overall Build Quality
In terms of build quality, consider the espresso machine material. This is usually stainless steel or plastic.
Stainless-steel machines are more durable, and it’s easy to clean them. Just wipe them down. However, plastic machines that are resistant to scratches and won’t break easily can be a good choice as well.
Machines with the best build quality will make usage easier. For example, the water tank, drip tray, and coffee puck container all have a large capacity and are removable. This means it’s easier to fill or empty them as needed.
Final Thoughts on How an Espresso Machine Works
I talked about all parts of an espresso machine — from a water reservoir, heating systems, and group head to portafilters and milk frother. Learning how an espresso machine works teaches you what’s important to consider when choosing an espresso machine and making great espresso.
Read more about our favorite espresso machines, and if you want to learn how to take care of your device, check out our guide on how to clean and descale an espresso machine.