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How I Stopped Worrying and
Learned to Love HXs
By Dan Kehn
Or, how to manage the brew temperature of your heat exchanger (HX) espresso machine.
The E61 is one of the most popular espresso machine designs among
home enthusiasts. Named after the year the patent was approved (a year
of an eclipse), its gorgeously sculpted and highly polished group is
recognized as a hallmark of fine espresso machines. The design of the
E61 group also has well-earned reputation as the heart of an espresso
machine that is forgiving of minor errors in barista technique. However,
knowing how to maximize this forgiveness necessitates a certain
understanding of how
heat exchanger espresso machines work, as will be
presented in the next section. If you're already familiar with
these types of espresso machines, you may want to skip ahead to the main topic of this
how-to, Managing HX Brew Temperature.
Espresso Machines 201: Heat Exchangers
Unless you repair home appliances for a living, there's a good
chance you don't really know or care what's under the covers
of your dishwasher. If you're like me, you have a basic
appreciation of what's going on. To the best of my understanding,
my dishwasher works something like this:
- Hot water and soap go in,
- A shower-marimba of hot water ensues for twenty or thirty minutes,
- Dirty water is pumped out,
- Clean dishes emerge.
This pretty much covers the full depth of my knowledge of our
dishwasher's inner workings. There's little motivation to
learn more because I don't see any practical use for acquiring the
knowledge. In contrast, it is worth understanding the principles behind
how your espresso machine works because this knowledge will guide your
preparation techniques and that definitely has practical
Whatever the espresso machine design, if the goal is to produce
espresso and espresso-based milk drinks, they all must address the same
- How to produce the coffee's optimal brew temperature water consistently, accurately, and
- How to produce powerful, abundant steam for frothing milk.
And as a consequence of the first requirement, all designs must also
- How to get the group to the desired coffee brew temperature.
If you're new to the espresso machine market, you may want to
Machines 101 before continuing. It briefly introduces three of
the most common espresso machine boiler designs, namely single boiler,
double boiler, and the focus of this article, single boiler / dual
which are commonly called "heat exchangers"
An intuitive appreciation for the workings of a single boiler
espresso machines is easy. They have two thermostats, one for brew
temperature and another for steam temperature. Flipping a switch chooses
which controls the heating element. Dual boilers are even more obvious;
they have one boiler for the coffee's brew temperature water and another for
Heat exchangers are an innovative, efficient design
that defies such a brief explanation.
To understand them more fully,
let's address each of the questions above as they apply to such espresso machines.
Thermosyphon circuit and
In essence, these requirements collectively pose the question,
"How do heat exchanger espresso machines work?"
How to produce brew temperature water consistently,
accurately, and reliably
espresso machines rely on a heavy, highly thermally
conductive metal group. Temperature-wise, think of the brew group as a
chunk of brass having almost the same weight as a bowling ball and the
few ounces of water for an espresso as an egg. This analogy really adds
meaning to the notion of "thermal mass," wouldn't you
The heat exchanger,
shown in the simplified schematic to the right as
the tube passing through the center of the boiler, is responsible for
warming the incoming fresh water to near the coffee's optimal brew temperature before it
reaches the grouphead. Once all that solid brass is at the desired brew
temperature, it acts as a dampener to either reduce the temperature of
the incoming water if it's a little too hot, or raise it if
it's a little too cool. Your goal is to get the group as close to
precisely the desired brew temperature as possible so it can "fine
tune" the somewhat volatile temperature of the water exiting
the heat exchanger.
This of course begs the important question that follows.
How to get the group to the desired brew temperature
The means by which the brew group arrives at the desired coffee brew
temperature depends on the espresso machine's design. Many commercial
machines rely on direct thermal conduction by attaching the group
directly onto the boiler. Other machines, like those we're
considering in this article, use a thermosyphon to circulate water from
the boiler through the group, as shown in the schematic to the right. As
the water in the heat exchanger portion of the loop warms (double
lines), it rises and flows towards the group (red arrow). The water then
cools and descends towards the bottom of the group (blue arrow),
returning to the boiler where it reheats and repeats the circuit.
How to produce powerful, abundant steam for frothing milk
The boiler itself is only partially full of water (or partially
empty, depending on your point of view). The space above the water is
filled with steam under pressure. Thus the boiler serves two purposes:
Providing steam from the top portion for frothing milk and an easily
regulated heat source to warm the water passing through the heat
Boiler pressure (bar)
Water temp (Fahrenheit)
The boiler pressure gauge indicates the amount of pressure, generally
between 1.0 and 1.2 bar (one bar = 14.5 PSI). An espresso
machine's boiler works under the same principle as pressure
cookers by increasing the boiling point of water to higher than the
212°F one expects when heating water in a pan on the stove. The
table to the left shows the actual water temperature at common espresso
machine boiler pressures.
The entire brew pathway heats up to some percentage of the boiler
temperature as this "super hot" water circulates between the
boiler and group. Since no system has 100% thermal efficiency, the final
grouphead temperature is determined by its own heat loss and that of the
copper tubing carrying water to it. In the case of most U.S. espresso
lovers, their target brew temperature is around 201-203°F; for some
countries overseas, the tastes are for a cooler 192°F. Manufacturers
adjust the length and diameter of the tubing making up the thermosyphon
loop to increase or decrease the efficiency of the circuit to arrive
approximately at the desired brew temperature.
Looking at the schematic, notice that the pump feeds into the same
thermosyphon loop. To simplify the diagram, I've omitted valves
and solenoids that direct water through the group and highlighted the
heat exchanger portion of the circuit passing through the boiler. For most semi-commercial espresso machines, the
heat exchanger is little more than a fat tube that traverses the boiler
with part of it immersed in the super hot water and the rest exposed to
The heat exchanger's job is to conduct some of the
boiler's heat to the water being pumped towards the grouphead.
One problem, however, is that being surrounded by water at no less than
250°F, it won't be long before the water inside
the heat exchanger
will also be super heated. Extracting an espresso using this
water will blast the coffee with blistering hot steam and assure an
extremely hot, bitter brew.
Enter the most important HX ritual you'll ever learn: The HX
cooling flush. The next section will introduce why, what, and how to
ready an HX
for brewing espresso. The essentials are covered in the
first few paragraphs. Experienced baristas may wish to continue reading
the rest of the section for a discussion of the thermodynamics behind
these efficient and fascinating machines.