How Espresso Machines Work
Which design is the right for your daily workhorse?
Which Espresso machine design is the right one for you? Since the first machines were built 120 years ago, there has been constant development and attempts to improve brewing quality. The market is full of products in the range from a handful of bucks up to the costs of a small car. At first sight, the sheer number of manufacturers and machines can be overwhelming.
However, if it comes down to machine design, manufacturers have settled on four different basic designs. All of them have proven themselves and all of them have their advantages and disadvantages. If you understand the basics, you’ll find it easy to decide on the machine design that suits your needs. Choosing a machine comes down to picking the one with the best finishing, useful improvements, or simply your favorite aesthetics.
The machine’s job
In a nutshell, an Espresso machine has only two basic tasks to accomplish to brew an Espresso:
- Precisely heat water to brewing temperature (usually ~94 °C) and consistently keep it there during the whole extraction
- Build up pressure (usually ~9 bar) and maintain it during the extraction process
With an unchanging input, the quality, and consistency with which it performs these tasks determine the quality of the extracted drink.
Apart from these two basic tasks the machine needs to provide steam to prepare milk-based drinks such as Cappuccino or the popular flat white.
There are a lot of other aspects when it comes to Espresso extractions, but necessary for a machine are only these three qualities. Everything else a machine can do is usually only to make your life easier by automating something you would otherwise have to do yourself (e.g. stop the extraction after a specific volume).
To achieve a good quality drink, the machine is only one part of the puzzle. If you want to know what it takes to brew a good Espresso, here is one of my other stories for you.
Single Boiler
The most basic and obvious design is called a Single Boiler. It works as you would expect from the name. It has a single metal boiler which is filled with fresh water. The boiler water is then heated to the desired temperature and used for Espresso extraction or to steam milk.
If pre-heated properly, this design tends to be relatively temperature stable and provide a good amount of steam power. As the temperature for different operation modes is controlled directly, it can be changed to adapt to different roasts.
Single Boiler designs are usually relatively simple and consequently can be serviced by oneself.
The most obvious caveat of this design is that it only consists of one water cycle. The portafilter water outlet and the steam nozzle are fed from the same boiler. Consequently, the boiler has to change the temperature from ~94°C (brewing) to ~130 °C (steam) and back. As such boilers usually have a volume between 0.4 l and multiple liters of water, changing between these operations can take some time or require to drain/refill the boiler. While this might be acceptable for brewing Espresso, it becomes cumbersome once more than 1–2 milk-based drinks are desired in sequence. Brewing Espresso and steaming milk simultaneously is not possible with this design.
Another trait of all-boiler based machine designs is the long pre-heating time of up to 30 minutes. During this time the boiler water and the brewing unit are heated to the right temperature. Although a “heating flush” (running hot water through the brewing unit) can reduce the pre-heat time, usually at least 15–20 minutes are required to achieve temperature stability during extraction.
As the boiler is never completely drained during normal operations or when turned off, some parts of the machine are always filled with water. This makes them prone to calcification, which damages affected components (like heating or valves). Due to the standing water, it’s also not so easy to do a regular pass-through decalcification, although for single boilers it is still possible. The most common approach to counter this is to use soft water only.
Dual Boiler
The Dual Boiler design is a consequent advancement of the Single Boiler idea to solve some of its problems. The name speaks for itself: there are two separate boilers. One is used for brewing water and constantly maintains brewing temperature. The other one is used for steam and therefore maintains higher temperatures.
The two separate boilers and water cycles provide very good temperature stability and allow for simultaneous brewing and steaming operations. Steam power tends to be very good as well.
As the temperature of each cycle is controlled directly, it is possible to change to brewing temperature and therefore adapt to different roasts. Not-needed cycles can be turned off (e.g. if you decide to only drink an Espresso today).
Although more complex as Single Boiler designs, the design is still relatively straight forward and can be serviced by oneself with basic knowledge.
In general, “Dual Boiler” machines are considered as the premium class of designs. The machines basically contain two machines in one and allow for a maximum of flexibility and comfort.
The boiler-based disadvantages listed before are valid for the “Dual Boiler” design as well. Long pre-heating times to heat the boilers and calcification due to standing water.
The premium class of this design also comes at a higher cost. “Dual Boiler” machines tend to be the most expensive ones on the market.
HX
The “Heat Exchanger” or “HX” design is a compromise between Single and Dual Boiler designs. It features two separate water cycles, but without the need for a second boiler. The one boiler in this design provides steam pressure and is used to heat a metal pipe that is routed through (the HX). Water flowing through this pipe is heated up precisely to brewing temperature.
The two separate water cycles allow for simultaneous brewing and steaming like a Dual Boiler, but save the costs for an additional boiler. Therefore the price is usually somewhere between Single Boilers and Dual Boilers. They also provide good steam power.
Because water standing in the HX will ultimately match the boiler temperature, these machines tend to overheat when idle for some time. Flushing the overheated water out of the HX before extraction (“cooling flush”) solves this issue.
The boiler-based disadvantages listed before are valid here as well. Long pre-heating times to heat the boiler and calcification due to standing water. HX designs usually allow now pass-through decalcification and have to be disassembled for decalcification. If used with soft water this only has to be done after a couple of years.
The main disadvantage of the HX design is that the steam boiler heats the HX by design. Consequently, the brewing temperature is depending on the temperature of the steam boiler. It can only be adjusted indirectly by changing the steam boiler temperature (or pressure as oftentimes the boiler is regulated by pressure control). This makes precise changes to brewing temperature more complex but works fine for almost all classic Espresso roasts.
Thermoblock
The Thermoblock design is an approach that does not rely on a boiler. It is based on a pipe spiral that runs through an aluminum block. This block can be heated very quickly and heats the water as it flows through.
One of the biggest advantages of Thermoblocks is the very short pre-heating time. The machines are usually ready in a couple of minutes and can provide brewing water indefinitely until the freshwater supply is empty. Also, steam can be provided continuously until the freshwater supply is empty.
The water temperature is directly controlled by the electronics heating the block which is in many cases designed to be adjustable. This means the temperature can be easily changed for different roasts.
Thermoblock machines are usually cheaper than HX or Dual Boiler and in the price range of Single Boiler machines.
Another advantage over boiler-based designs is the fact that water is only inside the machine when it’s actively operated. There is no standing water reservoir and therefore calcification is almost nonexistent. The machines can be operated with hard water and pass-through decalcified.
In classic Thermoblock designs with only one block, it is not possible to steam and brew at the same time. However, switching between both modes only takes a couple of seconds as the block is heated on demand.
A drawback of the design is that the steam produced in the Thermoblock has high humidity and the provided steam power is usually very low. Consequently steaming milk takes much longer than with boiler-based systems and the milk is diluted more.
Another aspect to be considered is that Thermoblock machines usually come with more electronics and more integrated components. That renders them more complex to self-service and spare parts are oftentimes more expensive.
Dual Thermoblock
Dual Thermoblocks are Thermoblock machines with two separate water cycles (and Thermoblocks). They allow simultaneous brewing and steaming.
Apart from that they come with the same advantages and disadvantages as classical Thermoblocks, but at a slightly higher price.
Comparison
As a conclusion let’s quickly recap all the strengths and weaknesses of the different machine types side by side:
As we can see, the decision on which machine design is the right one for you primarily depends on your everyday usage. There is no right or wrong here, you can achieve great results with all of these designs. However, some might be better suited for your needs and make your life a little easier. If you barely drink milk-based drinks, you can self yourself some money and be happy with a Single Boiler. If you drink a lot of milk-based drinks or will be caring for guests all the time, maybe an HX or Dual Boiler might be the better option. Thermoblocks are for the impatient, who want a decent result in no-time but can compromise on steam power.
