Double Boiler Versus Heat Exchanger

A commonly question raised by many is, “Should I get a double boiler or a heat exchanger?”.
In many cases, it depends on the user’s consumption level and expectations.

This is the double boiler.

A double boiler has two boilers on a single machine. Each boiler for a specific function. One is the brew boiler, set at a lower temperature. The other is a steam boiler set at a much higher temperature.

By keeping them separate, the user will have full control over the brew temperature and should be more consistent in the extraction.

This is the heat exchanger.

The heat exchanger has only one boiler set at steam temperature. It has another smaller “boiler” inside, a heat exchanger rod that passes through.

When an extraction is done, the pump will push room-temperature fresh water from the water source and pass it through the heat exchanger. By thermodynamics and hydrodynamics, the water heats up as the water travels through the heat exchanger rod. The water should heat to a therearound sweet spot, perfect for espresso extraction while it exits.

What’s the history between them?

Double boilers were invented later than heat exchangers. The popularity grew when the coffee conglomerates in the USA were on a campaign to open many cafes worldwide. They want the coffee machine to be more predictable and to have a standard SOP to make consistent shots of expected quality, even from amateur baristas. Thus, double boilers were invented.

Many early adopters believe that double boilers reduce the guesswork on whether the temperature is right. This is primarily thanks to the PID temperature regulator, a digital control introduced to espresso machines in 2005. Which can also be installed on Heat exchanger machines for similar benefits.

The difference in the temperature profile

Since we know that Double Boilers have a dedicated boiler specifically for brewing espressos. This means that double boilers should have a stable straight-line temperature profile. This means that if I set the brew temperature running at 90 degrees. The water temperature throughout the extraction should maintain around 90 degrees with little deviation from its set temperature.

Heat exchangers are expected to have fluctuations in temperature during the extraction process. Initially, idling water in the heat exchanger will warm up to a higher temperature. As you do an extraction, cold water goes into the heat exchanger to warm up while the hot water exits. This displacement of cold water entering reduces the heat in the heat exchanger. What will happen is the temperature of your extraction should have a gradual reduction in the later phase.

Here is an example of the heat profile comparing both systems

Taste difference?

Is a straight-line profile better in taste? Before that, we have to understand the definition of a good espresso. Who else drinks espresso faithfully in the world other than the Italians?

Here is their definition according to the Istituto Nazionale Espresso Italiano.

On sight, Italian Espresso has a hazelnut-coloured froth, verging on dark brown with tawny hinges. This cream has a very fine texture, which means that its mesh is tight and large or small bubbles are absent. The nose reveals an intense scent with notes of flowers, fruits, toasted bread and chocolate. All of these sensations are felt also after swallowing the coffee in the long lasting aroma that remains for several seconds, sometimes even for minutes. Its taste is round, substantial and smooth. Sour and bitter are well balanced and neither one prevails over the other. Astringency is absent or barely perceptible.

http://www.espressoitaliano.org/files/File/istituzionale_inei_hq_en.pdf

In short, the espresso should be balanced in its acidity and bitterness, with a lovely blossom of floral or fruity notes and nutty profiles. This may not appeal to everyone, but this is generally how coffee has been appreciated worldwide for decades.

From many taste tests, it’s not uncommon to hear that heat exchanger releases a broader spectrum of characters closer to the criteria. Why is that so?

It’s long known that pressure profiling machines, or spring lever espresso machines, produce the best-tasting espressos. These machines have a dynamic pressure profile. They are not designed to be stable, producing straight-line pressure profiles. They are known to express a broader spectrum of flavours and tame the harsher astringents in espresso.

These machines don’t have a uniform extraction pattern. It’s dynamic and usually has a declining pressure profile similar to the temperature profile in heat exchanger boilers.

That’s because heat and pressure can have a similar effect. The higher the heat or temperature, the more energy it brings to the coffee during extraction. And the more intense and bitter and less sour the espresso gets. If the heat is too high, it diminishes the delicate flowery and fruity aroma. So with a declining heat or pressure profile, it draws out a broader spectrum of flavours from the two ends. At the same time, balancing the bitterness and sourness as it doesn’t extract from a fixed point.

It is not a bad idea for a machine to have a declining heat profile found in heat exchangers. During extraction, the first few seconds will have a lot of oil from the beans extracted. In the oil are colloids with the effect of being receptors to bitterness. They bind and cut the number of astringents felt. That’s why Italians drink their espressos short, as ristrettos. They are smart enough to take only the nice part.

The second phase of the extraction, generally after the 15th second, is more about adding volume to make a bigger espresso than flavours. From this point on, the colloid from the lipid oil in the coffee starts to run out. Every bit extracted without the colloids increases the amount of astringent, which can overwhelm the flavours in the coffee. As such, a declining profile that brings less energy to draw the astringents in the later extraction phase may be beneficial, especially for a dark roast.

However, if your coffee is of a lighter roast or you use a lot of coffee for your extraction. Increasing the temperature, or having a straight-line profile, might have some benefits. The heavy dose of coffee you use may mean an increase of colloids to have the stamina to withstand a more prolonged extraction before releasing the astringents. Or the lighter roast will benefit a little from the bitterness for a better balance.

Does it mean anything about which heating system is superior in taste? Not really. It still depends on the coffee and your preference. If we have to put money into which machine should produce a better-tasting espresso. We believe the odds should favour the heat exchanger as most casual drinkers prefer darker roasts for the familiar thicker, nutty and stronger espressos. Compare this to light roast, which tends to produce espressos that are much more acidic and thinner in the body. The water used for extraction is also fresher in the heat exchanger, which should affect the espresso.

This should come as a big surprise as many assume that double boilers, being more expensive and used in many cafes, should be more capable of making better espressos.

Will adding a PID to a heat exchanger be useful?

We have a dedicated article on the PID temperature controller.

Some heat exchanger machines are equipped with a PID temperature controller to control the temperature digitally. It works by controlling the temperature of the steam boiler, which influences how hot the heat exchanger in the boiler will be. Adjusting the PID will increase or decline the temperature range used for extraction, influencing the espresso’s taste as much as a double boiler.

Double boilers have a straight-line temperature extraction, while heat exchangers have a declining heat profile. The temperature fluctuation with a PID is significantly reduced compared to pressure controllers. So there are still a lot of benefits.

The difference in water freshness

Water is the primary component of an espresso.  To Italians, many believe that fresh water is essential to a good espresso which certainly makes sense. Many experienced baristas believe that fresh water improves the clarity and expression of coffee.

Heat Exchanger

Every extraction draws fresh water from the water tank or water network. Water is always fresh from the water supply.

Double Boiler

Every time the machine gets activated, the pump will push some fresh water into the brew boiler, mixing with a large amount of stale water. The excess water gets pushed to the grouphead for extraction. In summary, the water used for extraction is mainly stagnant water that has been in it for some time. Especially if the brew boiler is large and usage isn’t heavy. You can’t drain out the brew boiler easily except by flushing a lot to displace the stagnant water with fresh water.

If you firmly believe that the water quality will heavily affect the extraction, then a heat exchanger should be your choice of machine.

Strength of the steam

The strength of the steam is directly related to the boiler’s size and the heating element’s strength. A bigger boiler means it has more space to store more steam and a stronger heating element means it can heat up faster. A heat exchanger typically has a larger boiler and uses a stronger heating element.

Comparing two espresso machines, a heat exchanger with a 2-litres boiler versus a double boiler with a 1-litre brew boiler plus a 1-litre steam boiler. The heat exchanger model will likely have much stronger steam power.

Maintenance and repairs

Typically, vendors will prefer to sell you a Double Boiler. Double boilers cost more and naturally come with more significant margins. Another big reason is that they usually have a higher frequency of failures, requiring more expensive repairs. And repairs are the most lucrative part of selling the machines.

A heat exchanger only has one boiler to maintain. With one heating element, overheating safety sensor, overpressure safety controller, low water sensor, safety valves, etc.

With a double boiler, the number of parts to maintain is almost doubled, and maintaining them will be both challenging and expensive. The chance of having something fail will increase with more parts involved. Doing basic things, like draining a boiler in a double boiler, is undoubtedly more work. Even a simple maintenance process like descaling is much more complicated on the double boiler requiring professional assistance.

A double boiler also has a longer water circuitry. This means troubleshooting will need to look into more areas and will be more tedious to find out which part has failed. This means more manhours, all charged to the customers. Maintaining a double boiler will be more costly, and the chance of going wrong is higher.

Startup time

Having two boilers, the double boilers will, of course, take a longer time to heat up.

If we talk about prosumer machines that can be used at home or in smaller settings, set within 13amps. A heat exchange E61 espresso machine can get properly heated up in about 20 minutes; a double boiler will require at least 30 minutes.

Although a double boiler has two boilers and two high-powered heating elements, the two heaters usually don’t heat simultaneously. If they do heat together, they will draw a lot of amperes and may trip the location if the ampere supply is insufficient. This is particularly true in the USA using 110V. Both heating elements can heat simultaneously for areas using 220V to 240V, but usually, there’s a limit to how much electricity can be used. As such, it won’t be much faster.

Also, since double boilers have more surface area for heat loss, they will have to try harder to heat up and compensate for the losses. As such, insulating the boilers in Double Boilers makes more sense, but insulation hinders heat transmission to the cup warming tray.

Usage

Both are used almost in the same way.

For the heat exchanger, you will need to flush some of the overheated water from the heat exchanger if it has been there idling for some time. Many Italian heat exchanger models are designed to be “cooler” than being too hot and require minimal flushes. You can tell that when you do your flushing and see if there’s a lot of steam in the release. Some machines, like the Bezzera BZ line, hardly require much flushing. They should not be flushed too much else you may cool the machine too much.

For double boilers, you will need to flush some water out of the grouphead. As you may want to clear the stagnant water retained around the grouphead. This also ensures that the valves and rods are uniformly hot. A lot of people believe that double boilers don’t need much flushing. This is only true under a few specific conditions. Like in high-traffic usage, the brew boiler is designed to be very small.

If stagnant water around the grouphead concerns the user. The user will also have to flush a lot more to keep the stagnant water in the brew boiler fresh. The amount to flush depends on the size of the brew boiler, so the brew boiler is usually designed smaller. However, the temperature could easily fluctuate during the extraction if designed too small. The user will have to accept one of the flaws in the design.

Is consistency guaranteed with a double boiler?

In terms of temperature control from the machine, yes, you are likely to be more consistent if you expect a straight-line temperature profile during extraction.

However, the complete picture of maintaining consistency in coffee making is a challenging task.  As the condition of your beans changes all the time. This can come from deterioration affected by light, heat, humidity and oxidation. Especially if you are in an environment where usage isn’t heavy.

Some variables are hard to maintain consistency. The roasters can’t get their roast consistently roasted to the same condition. The coffee roasting machine has retention after each roast. Most roasters won’t commit to cleaning that often.

The farmers can’t produce the same grade and size every time. The absorption of nutrients, the weather conditions and the watering of the coffee plantation are irregular. Fermentation and processing are hard to maintain constant. All these will more or less affect the coffee. 

In recent years, there’s been a trend of competition between manufacturers, adding more machine controls. Often, these additional features or controls are unnecessary or even confuse the user. Nor will they provide any significant impact. The only thing we know it can guarantee is machines full of additional components cramped into this narrow space. They are more likely to have something to fail.

So who needs a Double Boiler?

Double boilers do have certain advantages. You are likely to make more consistent shots, albeit better or worse. This might be important to a large cafe or a cafe chain where the priority on consistency is above everything else.

Imagine you are a large multinational cafe chain and need to integrate some procedures into the SOP for thousands of cafes worldwide. This precise control will be handy.

If you are a coffee roaster or a cafe and are stuck with an entire batch of coffee that you want to improve. You can use the PID control to explore the different profiles with different temperatures. Changing a few degrees up or down is unlikely to change the flavour dramatically. Changing your coffee beans supplier will be easier if they don’t meet your mark.

Some baristas may feel more confident looking at exact numbers if they have done things right. This can be done via the PID temperature controller. The correct way to know whether your coffee is made right is by tasting it, not looking at numbers, but this can’t be done if you have many stores as you can’t be everywhere.

Thanks for reading

Hope we have answered all your questions between heat exchangers and boilers; I hope this can assist you in finding the right system for your usage. The difference between a good coffee and a bad one ultimately depends on the barista.