What is a heat exchanger?
A heat exchanger (also referred to as HX) is an espresso machine with a single boiler that is dedicated to steaming milk. Rather than having a second boiler dedicated to brewing your espresso like on a dual boiler, it has a chamber with a coil attached to the boiler that rapidly heats water as it passes through.
What are the advantages and disadvantages of having an HX machine?
You'll often find the espresso machine in your favourite coffee shop is actually a heat exchanger, they are used as they have great steam power and the ability to make loads of drinks back to back. They are also great machines for home, you can simultaneously steam milk and pull your espresso at the same time all in a smaller footprint than a dual boiler and at a more stomach-able price.
When heat exchangers were first developed they didn't provide a way to control brewing temperature, which is seen as a disadvantage for a lot of home baristas. But they do guarantee to brew in a suitable range for espresso to ensure the coffee is extracted properly.
In recent years we have seen lots of HX machines emerge with either PIDs like on the Profitec Pro 500 and the ECM Technika V or a 3-position switch that controls the boiler's temperature, giving you a low, medium, and high setting like the Profitec 400, the ECM Mechanika VI slim and the Lelit MaraX.
These work by controlling the boiler temperature, which directly affects brewing temperature. For most, this is enough option and control. For others who want to play with temperature as a way to change how a coffee extracts, they have found that this still isn't precise enough.
Why buy a HX machine?
If you are in the market for a new espresso machine and don't want to break the bank with a machine the price of a dual boiler, but still want a great all-rounder machine for home then a heat exchanger is the machine for you.