You may be thinking, "I know how to flush a toilet. What is all this talk about 'flushing' an HX?" The previous section introduced the reasons behind the need to flush the heat exchanger before starting the extraction, but didn't answer the more important practical questions: How much water do you need to flush? Is it necessary before each extraction, or only if the machine has been idle? After how long an interval is the espresso machine considered "idle"?
Whew! You see why this initially appears to be a genuinely vexing problem. Let me assure you right now, the answers are quite simple: All you need to do is observe and listen carefully and you'll discover the technique is always the same—you've flushed enough water when it stops flash-boiling, plus a little more. To make this point clearer, see this video clip showing the temperature of the water exiting the group during a cooldown flush.
Notice how it sputters and spurts, then calms down to a steady flow right around the 24 second mark (Hint: use the single-pour spout rather than a double-pour spout to make the transition easier to discern). If the portafilter is loosely engaged, you can more readily hear the hissing of boiling water and steam—sound is a very accurate indicator of when the heat exchanger is cleared of overheated water. Flushing water out of the group until you've poured the same amount of "steady flow" water will give you the same starting temperature shot-after-shot. No fuss, no muss, and takes less than 30 seconds.
Comparing E61 espresso machines brew temperatures at different pressurestat settings
This simple approach to performing the cooldown flush applies to most prosumer E61 heat exchanger machines at any reasonable boiler temperature / pressurestat setting. To demonstrate this point, the chart to the right shows the in-basket temperatures during the extraction of a double espresso. I intentionally used three different pressurestat settings for the Andreja Premium, ECM Giotto Premium, and Isomac Rituale—as you see, using this technique produced almost identical profiles.
The steps of the complete ritual including the "water dance" cooldown flush are as follows:
Continue until the stream pours steadily for about two ounces (more or less depending on your taste) since the group is still over temperature when the hissing stops. It depends on the machine's pressurestat setting, but in general the water temperature is around 206-207°F when the last of the sputtering subsides. You can literally count "one-one-thousand, two-one-thousand, three-one-thousand" to tick off each degree from that point. To more easily judge when the heat exchanger has emptied of superheated water, keep the portafilter loosely engaged and listen for the sounds of hissing steam and boiling water.
If you've pulled a shot within the last five minutes, you'll need to pay extra attention because the cooling flush will be much shorter than if the machine has been idle for 10-15 minutes (on the order of ¼ the amount of water). You may hear only a brief hiss and that's it. Don't draw too much water or the next extraction will be under temperature (sour).
Don't dilly-dally; the water in the heat exchanger will be back to brew temperature in 30-45 seconds.
Recall that a few moments passed as you filled and tamped between the cooldown flush and the beginning of the extraction. This short interval gives the water in the heat exchanger enough time to rebound to brew temperature. I've found it is easier to consistently obtain the desired initial brew temperature by allowing a short natural recovery rather than adjusting the flush amount and starting the extraction immediately thereafter.
It may surprise you how little time is required for the HX to recover after a flush. That is the point to watch—if you wait too long, the first third of the shot will be all boiling water. You'll recognize it by the dark, oily crema ring that forms. Such an espresso is palatable served as a latte, since the extra bitter flavor adds "punch" to the drink that you may like, but they're pretty rough straight up.
The pour of most blends will begin dark brown and uniform, and then the stream color transitions to the point that "striping" develops. As the last of what you wish to extract is removed, the stream turns a pale blond. The last dribbles of an extraction are weak and bitter, so learn to stop the pump before blonding begins.
For the majority of home baristas, that's enough detail to produce very good espresso. The key points to retain are:
The "rebound time" is easy enough to integrate into your routine without much conscious thought. It simply means not dawdling in front of the grinder or flush-and-locking in rapid succession. Keep in mind, however, that this is specific to the prosumer E61s discussed in this article. The cooling flush requirements for other HX espresso machines, like for example the commercial LaCimbali Junior (review) with its huge HX, are quite different.
If the three prosumer HX machines mentioned above had different boiler pressure settings and yet their temperature profiles were similar, what's the "right" setting? For prosumer HX machines (e.g., Quickmill Andreja Premium, ECM Giotto, Fiorenzato Bricoletta, Grimac Mia, La Valentina, etc.), the boiler pressure setting is a tradeoff of the desired brew temperature, steam quality, recovery time between shots, flush amount, and flush frequency.
Most home baristas set their espresso machine's 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. I prefer the pressurestat setting on the lower end of the acceptable range 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 espresso 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 boiler pressure range is easier to manage temperature-wise than the upper end. 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. In contrast, a miscalculation of ten seconds risks producing an over-temperature extraction (very dark initial crema, black edges) for a pressurestat setting at the high end of the acceptable range.
The right boiler pressure for a prosumer machine is therefore the pressurestat setting that meets your drink preparation pace, and to some degree, your experience level. Finally, keep in mind that commercial HX machines in general are less influenced by the flush because of their heavier groups and larger heat exchangers. Manipulating the brew temperature outside of the "comfort zone" of such espresso machines 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).
I should warn you—espresso lovers are often perfectionists. If you're a "keep it simple" sort of person, the next section may seem a little over the top. On the other hand, if you're the type that revels in tweaking the last five percent out of every shot, read on.