Heat Exchanger vs. Double Boiler
A common myth for newcomers to the espresso
machine world is that one needs a double boiler machine to extract
espresso and steam/froth milk simultaneously (at the same time).
This myth is false. The following machines can extract espresso
and steam milk at the same time:
- Espresso machines with a
- Espresso machines with a double boiler ( independent coffee
boiler and independent steam boiler)
- Espresso machines with a indepedent coffee boiler and independent
We have worked on two generic visual representations - one for
a heat exchanger model and one for a double boiler model.
Immediately below is a visual representation of the heat exchanger
design with a thermosyphon grouphead. It is called heat exchanger
because there is a transfer of heat between the steam/how water
in the larger boiler vessel and the closed tube that passes through
In the first diagram, the heat exchanger tube is the larger tube
inside the single boiler with upside down 'V's. This is the water
that is flash heated to make espresso - incoming cold water is mixed
in with this hot water when the extraction starts. When not extracting
espresso, this same water inside the closed loop system is heated
in the heat exchanger and is pushed through to the grouphead which
forces the slightly cooler water back to the boiler for re-heating.
If the machine has a brew pressure gauge, this is one reason why
the gauge will fluctuate and change while the machine si idle. This
is why the brew pressure gauge should not be read while the machine
is idling and is nothing to worry about.
In some cases, the larger boiler is laid down horizontal, and the
heat exchange tube remains vertical. In some cases, the horizontal
boiler incorporates a horizontal tube from inside to another. In
most vertical boilers, the heating element is at the bottom of the
boiler. There are a few cases where the heating element is on top
of the vertical boiler with extensions internally to keep it under
water. We have never seen a vertical boiler with a heating element
on the side. On horizontal boilers, the heating element is usually
on the side.
In some cases where there is no thermosyphon tubes into the grouphead,
there is on one tube running to the grouphead. This grouphead is
usually attached directly/welded to the boiler for passive heating
by way of the metal.
Immediately, below is a visual representation of the double boiler
design with a thermosyphon grouphead.
Espresso quality in the cup
- The double boiler version has superior control over its heat exchange
counterpart in the temperature control of the water in the boiler.
This better temperature control allows the user to change the boiler
water temperature to match better with the coffee blend being used.
The superior temprature control offers a better extraction in the
quality found in the cup. Therefore, 1st-line 'considers' the PID
double boiler machine is a better choice for espresso-only consumption.
There have been some arguements made by some that the size of the
coffee boiler on a double boiler espresso machine can determine
the quality of the espresso in the cup. For example, there are some
who argue that a smaller boiler will have faster turnover of the
water, and this fresh water makes superior espresso. Although we
were a believer in this camp before we carried double boilers, we
no longer believe this theoretical model holds true. Our reasoning
is that if the smaller coffee boiler theory held true, then a heat
exchanger machine with a much smaller capacity would make extract
superior espresso over a double boiler machine.
Steaming capacity - From our observations,
the heat exchanger model is a more powerful steamer than its double
boiler counterpart, If frothing or steaming milk for milk-based
espresso beverages, the heat exchanger model is superior as the
heat exchanger model seems to have more steaming power and more
steaming capacity than its double boiler counterpart. .The reasons
- that the higher steaming performance of
the heat exchanger system design will allow quicker turnover in
producing cappuccino and latte drinks, and
- that the espresso quality is not 'that'
important as the milk in the drink dilutes the espresso and the
difference in the quality of the cup (milk and espresso) is very
Warm up time - The Vibiemme heat exchanger
model takes about 20-30 minutes to warm up, and the double boiler
version takes about 30-45 minutes to warm up both boilers. Our Vibiemme
espresso machines can be placed on a timer. However, the timer MUST
be plugged in the wall outlet first, and then a surge suppressor
(1080 joules or higher), and then either model can be plugged into
the surge suppressor. On the La Spaziale double boiler machines,
only the Version 2 can accept a specialized Spaziale timer. The
Version 1 units can not be on a timer at all.
Energy Savings - The double boiler version
allows the user to indepentently turn on/off the coffee boiler or
steam boiler. If the user only operates the coffee boiler, electrical
usage is only 600 watts for the coffee boielr heating element. The
heat exchanger has an one on/off switch for the entire machine.
Delta (difference or offset) of Temperature
between water temp in Boiler and Water temp at Grouphead
On average, the difference of the PID programmed temperature and
the temperature of the water at the grouphead is about 20 degrees
F. However, we have done a lot of testing with PID double boiler
machines in several different environments, and we can say in our
opinion with good certainty the following:
- if an offset is programmed in one environment, the offset is
most likely to be different in a different environment
- the offset can change even with the machine in the same environment,
but one of the variables change.
Environments include variables such as ambient temperature, drafts,
actual voltage to machine, etc. For example, we tested a PID machine
in a basement - one area where there was no draft, and another area
where there was a draft. The offset was different by as mush as
5 degrees. We tested a PID machine with a room temperature of 76
d F and then again with a room temp of 66 d F. Again, a different
difference in the offset.
- the offset can change if the PID sensor limescaled up - if the
PID sensor in the boiler is caked with limescale (and they have
a higher probability to do so on PID double boiler machines), and
we have seen up to a 40 d F differential between the grouphead water
and the programmed temperature.
Although some manufacturers use the offset as a selling point,
it can literally drive one nuts as this becomes a false pretense.
In our opinion, it is best to use the average of 20 when initially
programming the machine, and the best way to determine your optimum
taste is to change the programmed temp based ont he taste in the
cup leaving all other parameters is the espresso extraction process
being equal - just like one would only change grind settings with
all other things being equal. This is really the best way to get
the machine to the right temp for the coffee bean/blend that is
This is why we do not believe that offset programability is an
important factor for PID machines at this time.