there has been a lot of debates about the advantages and disadvantages of Heat exchanger espresso machine versus the double boilers espresso machines. There are merits to both systems with regard to temperature stability, steaming performance and user friendliness. Today, I will be looking at this decision to decide between a double boiler or a heat-exchanging espresso machine from a home owner perspective.
One of the highly touted feature of a double boilers system is temperature stability and double boilers system solve the issue by having two separate boilers each held at different temperatures. The bigger boiler is usually for steaming milk and is held at much higher temperature than the brew boiler, usually smaller in size and temperature are held at about 200F for brewing coffee. However, one of the biggest draw back to this double boiler arrangement is cost of both the machine and also the operating cost. For a home barista, the least expensive double boiler arrangement usually start at $2000 and going up (with the exception of the new Breville). The cost is justified because double boilers machines are more complicated, require an additionally boiler, heating element as well as additional control electronics. Along with this extra boiler, the cost of operating can be double as well with regard to electricity usage.
However, the newer breed of HX machines are becoming much more temperature stable. For instance, in the past two years or so, the Nuova Simonelli Aurelia were used in the World Barista competions. Before that, La Marzocco's double boilers machines were being used in these high profile competion. This is significant because it means that HX machine can perform on the same level (or better) as double boilers (in term of pressure and temperature stability, usability...etc).
Another critical criteria of HX espresso machine is the way brew water are heated to the temperature of approximately 200F. Fresh water are either drawn from the tank or the water line and heated to approximately 200F by the virtue of travelling through the heat-exchanger. In a double boiler, especially the larger volume machine, the water sit in the boiler for a long period of time and this can causes metallic taste to leach out compriomising the taste of the final espresso, this is especially a big concern at the usage volume of home barista. With HX espresso machine, this is not a concern because the size of the heat-exchanger tube is relatively small and the water passing through is fresh water.
So it seem like HX machine has many advantages for the home barista, size, power economy, and in some aspect also perfomance. What we need to see is manufacturers coming out with more temperature stable group head designs so that the home bariasta do not have to do too much to manage group head temperature.