We admit it, we're guilty. We thought that size did matter with regard to boilers on a semi-automatic espresso machine -- namely, that two boilers was better than one. The hierarchy in our mind was:
Single Boiler: From the Saeco Aroma to the Rancilio Silvia, the single boiler is a great little semi-automatic espresso machine that requires special attention to boiler temperature so that you're brewing well below the steaming temp and not burning your espresso. With a single boiler, you're not able to brew and steam at the same time -- we recommend steaming first, then brewing.
Heat Exchange: Instead of pulling your brewing and steaming water from the same vat, per se, heat exchangers like the Rocket Giotto Premium Plus or Quick Mill Andreja Premium transports fresh water from the reservoir through the boiler via a copper tube that is specifically designed in length and girth to heat the passing water to the optimum brewing temperature, not the steaming temperature. We are talking about a nearly 40F degree difference, so this improved temperature regulation significantly upgrades the espresso shot quality. This functionality also allows for simultaneous brew and steam.
Double Boiler: Only a few models on the market, such as the La Spaziale Mini Vivaldi or Izzo Alex Duetto, feature absolutely separate boilers for steaming and brewing, which allows you to maintain disparate temperatures and brewing and steaming at the same time. You can generally program your preferred brew boiler temperature on these machines and, in the home espresso machine space, they generally feature a quicker recovery time than their heat exchange counterparts.
So, based on those assessments, you'd understand why we were confused by the more is better idea -- that maintaining temperature is significantly easier when you've got two separate boilers doing their own thing.
However, in our recent research and education around the new line of commercial Faema machines we're now carrying, we learned that our hierarchical view was incorrect -- in fact, Italians haven't been using double boiler technology for decades, believing that the heat exchange technology provides for significantly improved espresso due to one major reason: It's alive!
Boiler water is considered 'dead' water because it's sitting in a little metal unit cooking away. Over time, this results in a significantly increased alkaline content in the water (ah yes, that lovely scale we keep talking about so much) and a mineral imbalance in extraction. Basically, the flavor's different.
Since heat exchange machines are continuously cycling fresh water through their siphoning system, they have an improved mineral balance and cannot become stale like the water in the double boilers might. So the flavor is significantly better and, therefore, preferred by connoisseurs the world over.
If you're in the market for a 'prosumer' machine, this is definitely important information for you to mull over. Not only is the footprint smaller on a heat exchange machine vs. a double boiler, but it just might pull a better shot.
