Single boiler, heat exchanger or dual boiler?

The difference is whether you can brew and steam at the same time, and how much control you have over each.

Single boiler

One boiler, heated to brew temperature (~93 °C) or steam temperature (~130 °C), but not both. You pull the shot, wait for the boiler to climb to steam temperature, texture the milk, then wait for it to cool before the next shot.

For one or two drinks that is a non-issue. For six, it is a slow morning. This is what almost every machine under about 600 euros is.

Heat exchanger (HX)

One boiler held at steam temperature, with a pipe running through it carrying fresh water to the group. Water passing through that pipe picks up heat on the way, so you get brew-temperature water and steam simultaneously.

The compromise is that brew temperature is not directly controlled — it depends on how long water has been sitting in the exchanger. Hence the “cooling flush”: run a little water through the group before pulling to dump the over-hot water. It is a routine you learn, and once learnt it works well.

Dual boiler

Two separate boilers, one at brew temperature and one at steam. Both are independently controlled, usually by PID. Brew and steam simultaneously, at exactly the temperatures you chose, with no routine.

This is the correct answer if you have the budget and make several milk drinks at once. It costs substantially more and takes longer to heat up from cold.

What to buy

One or two drinks a day: single boiler, and spend the difference on the grinder. Several milk drinks daily or guests regularly: HX or dual boiler, and dual if you drink light roasts where brew temperature matters.