In addition to how an espresso machine produces its required brew pressure, the method it uses to heat and maintain the water at the desired brew temperature distinguishes one design from another. The boiler type affects how the barista operates the espresso machine, so it's worth understanding how one type differs from another.
Boiler/Heat Shedding Group: This is the oldest system. Water is taken directly from the steam boiler at a temperature of roughly 120°C (250°F). The water's temperature drops to brew range in the group prior to its reaching the coffee. Most spring lever machines work in this way. Obviously, this is not a very precise way to regulate temperature. If the group is too cool, the final brew temperature will be too low; if it overheats, the final brew temperature will be too high. On commercial lever machines, shots have to be made at just the right pace to keep the group at the correct temperature; on many home lever machines, the machine has to be turned off after four or five shots and left to cool.
Heat Exchanger: Most commercial and larger home machines use this system. The heat exchanger is basically a pipe inside the boiler. As the water is pumped to the group, it goes through the pipe and heats up to brew temperature range. The average temperature can be adjusted by lowering or raising the steam boiler's temperature and pressure. Since the water arriving at the group is designed to be at the correct temperature, the group itself also has to be heated to the correct temperature so as not to change that of the brew water. This is done either by circulating hot water through the group or by bolting the group directly to the boiler. Again, this is not a very precise system, and it is difficult to adjust the temperature to a precise level. However, good engineering can make heat exchanger systems very stable, so that they hold the same temperature within 1°C to 2°C. This is mainly done by using very massive groups and heat exchangers. Once these are at the correct temperature, changes in the relatively small amounts of water going through them do not affect their thermal stability. However, they still depend on shots being made a steady pace. After a long idle time, the water in the heat exchanger will overheat, and the group may also drift to the wrong temperature. One has to go through a regime of flushing water through the group to get the system to the right starting temperature for making shots. The exact details of this regime vary from machine to machine, however the article How I Stopped Worrying and Learned to Love HXs offers general advice that can be adapted to most heat exchangers.
Heat exchanger systems have an important advantage compared to smaller single boiler home machines. They can steam milk and make shots at the same time, whereas single boiler machines cannot.
Single Boiler: Smaller home machines have a single boiler without a heat exchanger. When making espresso, one thermostat is used to heat the water to 90°C to 96°C; when steaming, another thermostat is used to heat the water to 125°C. There can be up to a one-minute wait for the boiler to switch from one temperature to the other. The major quality factor in these machines is the size of the boiler. The poorest machines have a thermoblock that heats less than an ounce of water on the fly. The best machines have boilers up to 25 fluid ounces. Although bigger is always better in terms of thermal stability, above about 12 to 16 ounces of boiler size, the added stability becomes somewhat academic compared to other factors.
In most home machines, the thermostat is a simple bi-metallic disc mounted to the outside surface of the boiler. These have a deadband (the range between turning on and off) of around 10°C. In order to get consistent temperatures shot-to-shot, you begin brewing espresso at the same point in the range, typically the moment it reaches maximum temperature. Varying the shot temperature is very difficult, and requires timing out the thermostat cycle precisely—a technique called temperature surfing.
Better home machines use vapor pressure or electronic thermostats which measure the water directly and have narrow deadbands. Although these are adjustable in theory, in practice, they are not very accessible.
Many home espresso enthusiasts take one of the better single boiler home machines and use industrial temperature controllers (called PID controllers) to precisely regulate the temperature. When this is done, these machines deliver very repeatable and adjustable shot temperatures.
Double Boiler: Single boiler machines cannot steam and pull shots at the same time. However, with the right controls, they deliver very precise temperature control. Commercial manufacturers took note of this and are producing double boiler machines. These have one boiler used for brewing and another for steaming, with each boiler set to the correct temperature for its function. In theory, such machines can deliver completely stable temperatures. In practice, groups are not regulated and shot temperatures can vary as widely as on the better heat exchanger models. Nevertheless, since these espresso machines almost always have accessible electronic controls, they are much easier to set to the desired temperature than any other kind of machine. Also, this is an active area of innovation, and double boiler espresso machines are becoming more precise with each new model iteration. The most advanced current models claim to keep temperatures within 0.5°C of the setting under all operating conditions.
There are several models of double boiler machine now available for the high end home market.
It depends. Most home espresso enthusiasts buy small commercial machines, or do their own upgrades on better home models, not because they drink more coffee, but because they are looking for consistency. Consistency in pressure and temperature has three aspects:
You will pay more for beautifully designed cases, for higher quality, long life components, and for good workmanship and maintainability. Inexpensive home machines have almost become disposable, and such features are likely a waste of money. More expensive machines are like major appliances—their lifetime is measured in decades, and they are designed to be repaired and serviced. Since it is more convenient to do routine maintenance and service yourself, a well designed machine with high quality components greatly increases the joy of ownership. Since such machines are long term fixtures in the home, the quality and type of case design should be chosen with their location in mind. It is rather odd to walk into a beautiful marble tiled kitchen and see its granite counters populated by cheap plastic gadgets; on the other hand, a machine destined for placement behind a counter or in a working kitchen can be simple and utilitarian in design.
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