Boiler Types for Espresso Machines
What determines if an espresso machine can brew and steam at the same time? The type of boiler is responsible for that. There are several different ways in which the boiler on the espresso machine can work. To understand the various boiler designs you need to understand that brewing espresso requires different temperature than steaming and frothing milk. The ideal brewing temperature is between 195°F and 205°F. The temperature required for steaming is at least 240°F.
Single boiler / heat dissipated by grouphead
This design is typical for a manual espresso machine. The water in the boiler is heated up to around 250°F. When you brew the espresso you rely on the grouphead to dissipate enough heat to cool down the water to the right brewing temperature.
The main drawback of this system is that when the grouphead gets overheated the coffee gets burnt. In practice this means that you can usually make only 4 or 5 shots and then you have to turn off the machine and let it cool down. On the plus side you can brew and steam simultaneously with this system.
These machines do not have a boiler as such. Instead they use a thermoblock or thermocoil system to flash-heat the water on demand. This can heat up the water very quickly. But unfortunately it quite often does not heat up the water high enough. The thermocoil can also build up scale quite quickly. I would advise against buying a machine with a thermoblock.
This is a very common design for the semi-automatic and automatic espresso machines. The machine has only one boiler. It has two thermostats: one for brewing and the other one for steaming. When you make espresso the brewing thermostat makes sure that the water is at the correct brewing temperature. When you then want to froth the milk, the water in the boiler has to be heated up to the temperature set for steaming. So with these machines you cannot brew and froth milk at the same time.
Single boiler with heat exchanger
These machines have a single boiler that is set to the steam temperature. The heat exchanger is a coiled tube that goes from the water reservoir to the grouphead through the boiler. As the cold water passes through the boiler it gets heated up to the brewing temperature. So the advantage is that you can brew and steam at the same time.
One thing to be careful about with this system is the build up of scale. If you fail to descale the machine regularly then the scale inside the heat exchanger will build up and may eventually completely block the coil. Any built-up scale inside the coil is very difficult to get rid off. So bear this in mind especially if you have hard water.
In here there are two separate boilers - one for brewing and the other one for steaming. So you can brew and steam at the same time. You can usually control the temperature really well. On the downside, these espresso machines are quite expensive.
Further ReadingHome Espresso Machines
Espresso Machine Types
Manual Espresso Machines
Pump Espresso Machines