What is the right pressurestat setting for an HX espresso machine?

[HB]

My contribution for today is an answer to a frequently asked question, specifically one that might naturally follow after having read the article How I Stopped Worrying and Learned to Love HXs:

If the three prosumer HX machines mentioned in the how-to had different pressurestat settings and yet their temperature profiles were similar, what's the "right" setting?

For prosumer HX machines (e.g., Andreja Premium, Giotto, Bricoletta, Mia, Valentina, etc.), the pressurestat setting is a tradeoff of the desired brew temperature, steam quality, recovery time between shots, flush amount, and flush frequency. I generally set mine as low as possible (1.0 bar) but that varies from machine-to-machine. Most set their pressurestat somewhere between 0.8 and 1.2 bar (measured at the top of the cycle), and then determine the flush amount to bring the group to the target temperature. The pressurestat setting is therefore just one part of the equation. But how does this setting correlate to temperature? Recall the following table shown in the article:

• Pressure (bar) = Boiler's water temperature (Fahrenheit)
  1.0 bar = 253F
  1.1 bar = 255F
  1.2 bar = 257F
  1.3 bar = 259F

As you can see, there may be as much as a 4 degree swing just in the pressurestat deadband (difference between on and off, e.g., 1.0-1.2 bar). The actual brew temperature variance is less because the heavy group heated by the thermosyphon compensates for a bit of the fluctuation, acting as a dampener for the short-lived rise and fall of the boiler's temperature. I like the pressurestat setting low because it slows the overheating of the water in the heat exchanger. The drawbacks are that it diminishes recovery time and especially steam production, some machines to the point where they no longer can create microfoam well. But if you're preparing drinks only for yourself and perhaps a couple friends, the lower end of the spectrum (1.0) is easier to manage temperature-wise than the upper end (1.2+). The barista's job is easier in the former case because the rebound time is long enough that the difference in brew temperature between a delay of 15 seconds and 25 seconds after the flush and the beginning of the extraction isn't dramatic; at a pressurestat setting of 1.2 bar or higher, a miscalculation of ten seconds risks producing an over-temperature extraction (very dark initial crema, black edges).

The "right" setting for a prosumer machine is thus the pressurestat setting that meets your drink preparation pace, and to some degree, your experience level. However keep in mind that commercial HX machines in general are less influenced by the flush because of their heavier groups and larger heat exchangers. Futzing with the brew temperature outside of the boiler's "comfort zone" by flushing requires more attention to timing the length of the flush and the rebound time, which compared to prosumer machines is very short (e.g., 10 seconds or less).