A heat exchanger machine has a single boiler held at steam temperature — around 120 to 130 °C — with a pipe running through it that carries fresh water from the reservoir to the group head.
Water passing through that pipe picks up heat on the way. So you get brew-temperature water and steam simultaneously, from one boiler.
The advantage
Brew and steam at the same time, which a single boiler cannot do. For anyone making several milk drinks, that removes the wait entirely.
Cheaper and simpler than a dual boiler, and it heats up faster.
The compromise: the cooling flush
Brew temperature is not directly controlled. Water sitting in the exchanger between shots keeps absorbing heat, so the first water out is too hot.
The routine is to run a little water through the group before pulling — the cooling flush — until the temperature is right. Owners learn their machine’s flush by feel and by sound.
It works well and it is a routine, which means it can be got wrong when you are half awake or when someone else uses the machine.
Versus dual boiler
A dual boiler controls brew and steam temperatures independently, usually with a PID, and needs no flush. It costs more and takes longer to heat.
Who it suits
Someone making several milk drinks a day who wants prosumer capability without dual-boiler money, and who does not mind learning a routine.
Last reviewed 8 August 2026