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
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
HX cooling flush made easy
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
The steps of the complete ritual including the "water
dance" cooldown flush are as follows:
- Draw water through the group with the portafilter in place.
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).
- Remove the portafilter, fill and tamp the basket.
Don't dilly-dally; the water in the heat exchanger will be
back to brew temperature in 30-45 seconds.
- Lock the portafilter back in.
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.
- Watch the pour, looking for the color of the stream to lighten.
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:
- Draw water until the water dance ends, plus a little more to taste
- Allow a short recovery time after the cooling flush.
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.
What is the "right" boiler pressure setting?
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
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.