Single Boiler vs Heat Exchanger / HX vs Double Boiler Espresso Machine Comparison
5th Dec 2017
Hi I’m Joe Kolb from Espresso Outlet. Today I’m going to be talking about the differences between single boiler, heat exchanger and double boiler espresso machines. As well as which boiler type is best for your needs.
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I’ll start by talking about how each boiler system works, then talk about the pros and cons for each.
Single Boiler – How it works
Boiler fills about half full with water
The machine warms up, stabilizing temperature in the boiler and the group head. Depending on the machine/settings, the temperature at the brew group will be somewhere between 198 degrees and 204 degrees.
If the machine has a PID, the brew temperature can be adjusted and maintained accurately
When shot is pulled, water is pulled directly from the boiler. Temperature remains stable (how stable depends on the machine)
After shot is completed, the machine can be switched to steaming mode. Before steaming can take place, the boiler temperature has to increase to provide enough steam pressure for frothing milk. At about 255 degrees, the machine is ready for steaming.
On most machines, the increase in temperature for steaming indirectly raises the temperature of the grouphead.
After steaming is completed, the steam functionality on the machine can be switched off, allowing the machine to cool back down to brew temperature.
While the temperature can be pretty stable for brewing the first shot, the temperature for any additional shots won’t be as stable due to the increase and decrease of temperature for steaming. This creates flavor flaws in the espresso.
Single Boiler – Pros and Cons
Simple design with less parts than heat exchanger or double boiler machines
Faster warm up time
On machines with PID, ability to adjust brew temperature independently of steam temperature
No cooling flush required on some models, such as those with PID controllers. Others may require temperature surfing.
Wait time between brewing and steaming
Less temperature stability of brew temperature, especially when making multiple drinks
In general, single boiler machines have smaller boilers with less steam pressure
Heat Exchanger – How it Works
Cold water from the water source is pumped into the boiler.
The water in the boiler is heated to steam temperature, or about 255 degrees
Steam and water from the hot water tap comes directly from the steam boiler.
When a shot is pulled, water for brewing goes directly from the cold water source to the brew group. The steam temperature boiler has a tube inside of it that the brew water flows through. As it’s flowing through the tube it is flash heated to brew temperature, or about 203 degrees.
After a brew cycle is completed, water that remains in the tube will eventually increase to steam temperature. Because of this, a cooling flush is required prior to pulling a shot.
A PID temperature controller or P-Stat on a heat exchanger controls the temperature of water in the steam temperature boiler. Since water for brewing is flash heated as it flows through a tube, they will not work for directly adjusting brew temperature, however brew temperature will indirectly raise or lower depending on the temperature setting of the steam temperature boiler.
Heat Exchanger – Pros and Cons
A heat exchanger uses less energy when compared to a double boiler, since only one boiler has to be kept at a constant temperature
Water for brewing comes directly from the water source vs coming from a boiler like on a single boiler or double boiler machine. This means water for brewing is fresher and can result in better flavor, especially if boilers have mineral deposits.
Since only one boiler is being heated, recovery and warm up time on a heat exchanger machine are quicker than on a double boiler machine, especially on a 15 amp 110 volt connection. It should be noted that some home use double boilers have 20 amp connections which makes the recovery and warm up time about the same.
Since there are fewer components in a heat exchanger, they generally cost less than comparative double boiler machines.
Maintenance and repairs on a heat exchanger tend to be less than on a double boiler.
Temperature adjustment of brewing and steaming are not independent. If you lower the steam temperature, the brew temperature is also lowered. If you raise the steam temperature, the brew temperature is also raised. This may not be ideal for getting the desired steam pressure and the desired brew temperature at the same time. As an example, if you want more steam pressure and raise the boiler temperature, the increase in temperature may not be ideal for the type of bean you are using, which could alter the flavor of the shot.
While adjusting the temperature of the boiler also adjusts the temperature of the brew boiler, the adjustments are less precise that on a single or double boiler and can make it more difficult to fine tune flavor
Double Boiler – How it Works
Cold water from the reservoir or direct water line connection is pumped into both boilers to fill the boilers.
Water in the steam boiler is heated to steam temperature. Water in the brew boiler is heated to brew temperature. Generally, the temperature of each boiler can be independently adjusted.
After a shot is pulled, the temperature of the brew boiler and grouphead generally remains stable and is usually ready for another shot to be pulled right away.
After milk steaming has completed for one drink, there may be some recovery time depending on the machine, but in general, milk for another drink can be steamed without much or any downtime.
Double Boiler – Pros and Cons
•Independent brew temperature and steam temperature adjustment. Very easy on machines with PID
•Less effort required to get consistent brew temperature shot after shot
•No cooling flush needed
•Brew and steam at the same time or with no wait between brewing and steaming
•More complex design with more parts needed
•More expensive than comparable single boiler or heat exchanger machines
•Water for brewing is pulled from the boiler vs directly from the water tank
•Longer warm up time compared to single boiler or heat exchangers with same amperage.
As a general rule of thumb:
A single boiler is a good machine to get if you primarily drink plain espresso and only a limited number of milk based drinks – as an example 90% espresso. If you tend to change your beans often, then a machine with a PID temperature controller is important. Also if you are the only person in your household that makes espresso, a single boiler can work fine.
A heat exchanger is a good machine for someone that drinks primarily milk based drinks and for someone that tends to use the same beans over and over again (limiting the need to adjust brew temperature).
A double boiler machine is good for someone that does a lot of milk based drinks and a lot of plain espresso. If you change your beans often, use a lot of delicate single origin beans and have a sensitive palate, a double boiler is your best option due to enhanced temperature stability and control.