There are several popular espresso machine designs, each with a fundamentally different method of handling the production of two temperature settings*. Single boiler, dual thermostat machines allow you to switch between brew temperature and steam temperature at the touch of a button... but can take several minutes for the boiler to stabilize at the new temperature. Although single boiler espresso machines may function very well for straight espresso shots (no milk), they are marginal for producing even one milk-based espresso beverage. Try to make cappuccinos for a crowd, and you're better off with another design.
Lever machines also have a single boiler, but typically only one thermostat setting. The boiler is kept at steam temperature, always ready for frothing milk. Brew water is magically cooled to the right temperature by contact with the cooler group head. The disadvantage? After a couple of shots, the group head heats up, and can no longer function appropriately as a heat sink.
Heat exchanger (HX) machines, like levers, have a single boiler that is kept at steam temperature. Fresh, cool water is drawn through a heat exchanger and flash-heated to brew temperature. The heat exchanger is a thin tube of metal that runs through the boiler. The large volume of hot boiler water, typically at least ten-fold greater than the volume of the heat exchanger, heats the brew water on the fly as the shot is pulled.
Double boiler (DB) machines take a brute force approach to temperature management. Need water at two different temperatures? Fine, let's just put two separate boilers in the machine. The brew boiler is kept at brew temperature, and the steam boiler at steam temperature. Primary disadvantages are additional cost and power requirements.
Most high-end prosumer and commercial machines use either the HX or DB design, because these allow you to crank out one milk-based espresso drink after another. So which is better? This question has become more relevant with the introduction of several new DB models during the last year. And so the debate rages on, occasionally reaching heights of absurdity. In a recent CG thread, one DB proponent flatly proclaimed:
For the past few weeks, I have been in a position to compare the performance of a pair of prosumer espresso machines, one HX and one DB. These are the Quick Mill Vetrano and the La Spaziale Vivaldi S1. Pricing on these machines is similar (close to $2K, with the S1 typically costing a few hundred dollars more than the Vetrano). Both machines won CoffeeGeek Editor's Choice Awards in 2006. Both are fully plumbed rotary pump models.ANY dual boiler machine beats ANY HX, hands down.
But there the similarities end. The Vetrano uses the classic E61 grouphead: 58mm in diameter, heated by a thermosiphon, with lever-activated preinfusion. The S1 uses the La Spaziale grouphead: 53mm, heated by conduction from the boiler, and no preinfusion (preinfusion has been added in the S1V2).
Despite these fundamental differences, I thought it might be interesting to share some of my observations on these two machines. Take them for what they are worth: hearsay evidence, nothing scientific whatsoever. A valid comparison of HX and DB designs would require paired machines with the same grouphead, differing only in their use of a heat exchanger vs. dual boilers, and a double blind taste test to generate the data. Ian (cafeIKE) is already in a position to do this with his VBM HX and DB models. If someone wants to loan me, say, an Izzo Alex (HX) plus Izzo Duetto (DB), I'll be glad to report my findings.
Getting back to the Vetrano-S1 comparison:
First and foremost, both of these machines are capable of making truly excellent espresso. Forget the "hands down" hyperbole; that simply does not hold water. Earlier this week, a friend came over for a tasting session. After pulling several paired shots, he confirmed my impressions of the past few weeks. The pours are subtly different on the Vetrano and S1. The S1 shots taste slightly sweeter and flatter. The Vetrano shots are brighter, more complex, with a wider range of flavors. We both liked the S1 for straight shots, but preferred the Vetrano for Americanos and cappuccinos. Again, this is entirely consistent with my general impressions over the several weeks, using different blends, temperature settings, and brew ratios.
How much of this taste difference is attributable to the HX vs. DB design? Well, that's an interesting question. You could certainly argue that the Vetrano's preinfusion or larger diameter baskets are responsible for taste differences, but I believe it's more likely to be brew temperature differences. The S1 should be providing brew water at a fairly constant temperature. The Vetrano's grouphead thermometer (recommended) shows a typical "humped" HX brew temperature profile. The wider range of HX extraction temperatures may fully account for the wider range of flavors in the cup.
Some other observations:
* Brew temperature management is easier on the DB. But it's not all that much easier, especially with a grouphead thermometer on the Vetrano.
Huh? What about the dreaded HX flush? Guess what, folks: you need to flush both machines for optimal results. Let me repeat that: you need to flush both machines for optimal results. Two 2-3 oz flushes are recommended when the S1 has been sitting idle, to bring the cooling grouphead back up to brew temperature. Only one flush is needed for short idle periods, and none if you are pulling one shot after another. The Vetrano requires a cooling flush after sitting idle, to eliminate overheated water that's been cooking in the heat exchanger. Flush volume? Typically 3-4 oz. In fact, I often found myself flushing more on the S1 than the Vetrano.
However, the DB flush volume is relatively unimportant, whereas it's critical to monitor the flush volume closely on an HX machine. This is not difficult, but it does require paying attention to either the water dance or the grouphead thermometer.
* Reproducibility is easier on the DB. The S1 (presumably) produces a flat brew temperature profile, pour after pour. Nailing the same humped profile on the Vetrano is a hit-or-miss proposition that depends on the idle period, flush volume, and delay prior to pulling the shot.
* Pulling shots for a crowd is easier on the S1. Just wait for the (small) S1 brew boiler to come back up to temperature (30-40 seconds after the previous pour) and you're ready to go. Recovery time on the Vetrano is longer. The (large) S1 steam boiler is capable of frothing large quantities of milk in short order, and milk frothing has no effect on espresso brewing. The smaller Vetrano boiler is great for frothing small quantities of milk, but is slower on large volumes... and heavy use of steam will require a recovery period before brewing espresso.
* The HX design lends itself very nicely to experimenting with different brew temperatures. Sure, you can program in a new brew temperature on the S1... and then wait for the machine to stabilize at the new temperature setting.** To change brew temperature on the Vetrano, all you need is a longer or shorter flush.
* Personally, I prefer the 58mm brew group. There have been claims that the narrower, taller Spaziale baskets reduce channeling. I see no evidence to support this claim. In my experience, taller baskets (like 58mm triples) are more susceptible to channeling, not less. There is also a much wider range of options in filter baskets in the ubiquitous 58mm diameter. Whenever I use the S1, I dearly miss my Synesso ridgeless doubles.
* The lack of insulation on the S1 steam boiler is, IMHO, inexcusable. The uninsulated brew boiler, only slightly less so. The saving grace (if you remember to do this): it's very easy to shut off the steam boiler when not in use. The boiler comes back up to pressure in just a few minutes.
These observations may not generalize to other HX and DB models. Some commercial HX machines with large boilers and heat exchangers appear to have excellent brew temperature stability. Other popular prosumer DB models feature the E61 grouphead (which may or may not be optimal for a DB). The large steam/small brew boiler configuration on the S1 makes sense to me, but perhaps a large PID'd brew boiler has even better thermal stability.***
So there you have it: some thoughts on HX and DB machines, and a first cut at comparing two popular HX and DB models.
Bottom line: I like both of these machines very much. Deciding which one to keep in my kitchen will be a difficult choice.
* Pardon the generalizations in these machine design descriptions. I'm well aware that there are several different variations on these themes.
** The S1 user interface for programming brew temperatures and water volumes was designed by a demented, evil, sadistic mind.
*** Coming from the lever/HX universe, I find ultra-precise brew temperature stability to be overrated.