Espresso needs water at around 93 °C. Steaming milk needs steam, which means water at 120 °C or above. One machine, two temperatures — how a machine resolves that is its boiler configuration, and it is the main thing separating one price tier from the next.
Single boiler
One boiler, one temperature at a time.
Pull the shot at brew temperature. To steam, the machine heats the same boiler to steam temperature, which takes a minute or two. To pull another shot, it has to cool back down — and cooling is slower than heating, so you run water through until the temperature drops. This is called a cooling flush, and it is a routine part of owning one of these machines.
Practically: fine for one or two milk drinks, or for anyone drinking espresso without milk, where the steam side never comes up. Tedious for four drinks in a row.
Worth knowing: if you drink black espresso, a single boiler is not a compromise at all. You are paying for capability you would never use. Some very good machines are single boiler, and the money goes into temperature stability and build instead.
Heat exchanger
One boiler held at steam temperature, with a tube passing through it. Brew water travels through that tube on its way to the group head, picking up heat in transit.
The result is steam permanently available and brew water heated on demand. You can steam while a shot pulls. No waiting, one boiler.
The catch is that water sitting in the tube between shots overheats. So a heat exchanger machine needs a cooling flush before each shot — run water through the group until the overheated slug has passed. A few seconds, but it must be learnt and it must be done, and the right duration varies by machine and by how long it has been idle.
Practically: a real jump in convenience over single boiler, at a real jump in price. Brew temperature is less precisely controllable than a dual boiler, because it is a consequence of the steam boiler’s temperature rather than an independent setting.
Worth knowing: heat exchangers have a long track record in home machines and many are excellent. The flush routine is the thing people either accept quickly or find irritating forever.
Dual boiler
Two separate boilers, each held at its own temperature. One for brew, one for steam.
No waiting, no cooling flush, and brew temperature is set independently — which matters if you change between roasts, since lighter roasts generally want hotter water.
Practically: the configuration that removes the compromises, at the price that implies. Also physically larger and slower to reach temperature from cold, since there are two boilers to heat.
Worth knowing: the independent brew temperature is the real advantage, more than the simultaneity. If you drink one kind of coffee and make one drink at a time, you may be paying mostly for convenience you do not need.
Thermoblock and thermocoil
Common on entry machines. Rather than a boiler holding a reservoir of hot water, water flows through a heated block and is warmed as it passes.
Heats fast, which is the appeal — ready in under a minute. Temperature stability across a shot is generally worse, because the block is heating water in real time rather than drawing on a stable reservoir.
Some modern implementations with good control are much better than the category’s reputation. Others are the weakest part of an otherwise reasonable machine.
PID, and why it appears in every listing
A PID is a controller that holds the boiler at a set temperature rather than letting it swing between a high and low cut-off. Without one, boiler temperature oscillates by several degrees, so identical shots pulled minutes apart come out differently.
PID is available on machines in every configuration above and is genuinely worth having. It is also the specification most heavily advertised, which sometimes obscures that the boiler configuration matters more. A PID-controlled single boiler still cannot steam and brew at once.
How to choose
Black espresso, one or two a day. Single boiler with PID. Spend the difference on the grinder.
One or two milk drinks a day, and you are patient. Single boiler works. You will wait between the shot and the steam.
Milk drinks regularly, or more than one person. Heat exchanger or dual boiler. This is the case those configurations exist for.
You change roasts often and want temperature control. Dual boiler.
Speed from cold matters more than anything. Thermoblock, accepting the stability trade.
The short version
The boiler configuration answers one question: what does the machine do while it is doing something else. If you never need it to do two things at once, the expensive configurations are solving a problem you do not have — and the money buys more coffee if it goes into the grinder instead.