A few years ago, high-quality espresso machines were fitted with copper or brass boilers, but recently there have been an increasing number of suppliers who are installing stainless steel boilers. First, some basic information about copper and stainless steel - brass is a copper alloy and is used less frequently.
Portafilter machines with copper boiler - proven for many years
Compared to stainless steel, copper has a very high thermal conductivity . This means that copper conducts heat very well. Copper has therefore always been used for top quality cookware. When combined with water, a copper kettle quickly forms a protective layer of copper oxide, a water-insoluble layer that protects the copper and has a completely neutral taste.
Copper kettles should not be descaled with a strong acid in order not to damage the protective layer. Copper as a mineral is a very important trace element that the human body needs. Our modern water pipes are also made largely of copper. Traditionally, copper kettles are preferred for thermal and mechanical reasons - it is no coincidence that the founders of modern coffee machines such as Guiseppe Cimbali were coppersmiths. To make copper even more resistant, it is usually coated on the inside. Mineral deposits such as limescale have a harder time settling and impairing the function of the machine.
Portafilter machines with stainless steel boiler - the newcomer
Stainless steel is very inert both thermally and chemically and is therefore insensitive to mechanical and many chemical influences. Stainless steel is tasteless and very pressure-resistant. Modern high-performance machines therefore increasingly have stainless steel boilers. However, stainless steel is much more difficult to process because it requires complex welding. Its high resistance to deposits makes it a good boiler material. However, stainless steel also has rather poor thermal conductivity, so that the heating output measured by volume is often higher than with comparable copper boilers.
Copper boiler vs. stainless steel boiler:
Stainless steel is particularly impressive in large-volume boilers. Here, the heating (suspended in the water) is controlled directly by the boiler pressure via a pressure switch (pressure valve) and responds accordingly well. Stainless steel is less sensitive to the water components and is easy to clean of mineral deposits. However, copper scores points for faster heat dissipation from incoming fresh water and is always the first choice for dual-circuit systems with heat exchangers, which heat the fresh, oxygen-rich water for making coffee. Here, copper means shorter routes and faster reaching of the target temperature while at the same time requiring less energy.
However, in modern machines controlled by temperature sensors (eg portafilters with PID systems), the boiler temperature is measured directly at the boiler. Here too, the good heat-conducting copper boilers have an advantage. Even small temperature fluctuations are passed on to the thermostat via the copper boiler and the heating can quickly and sensitively adjust the temperature.
repairs
Boilers are particularly stressed at the transitions to the heating, the valves or the heat exchangers. If repairs are required after many years of operation, copper boilers are easier to repair than stainless steel boilers. Leaks in copper can be soldered and the individual adjustment of pipes can also be done quickly (in our specialist workshop). If there are leaks, stainless steel boilers are usually completely replaced or have to be sent to the manufacturer for repair.
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Conclusion
Both materials are very suitable as boiler material. Copper absorbs heat very well and transfers heat to the water more quickly. This is an advantage, especially for dual-circuit boilers with a thermosphon. Stainless steel, on the other hand, is very pressure-resistant, tends to form fewer deposits and is somewhat easier to remove, although this also applies to specially coated copper boilers. Copper is easier to work with than stainless steel, but the pure material value of copper is higher. In terms of price, the boiler material therefore generally makes no difference.
For us, copper kettles are still the first choice in the semi-professional sector. Stainless steel is just as good in terms of basic suitability and quality. The decisive factor is whether you like the machine and how it works. In the professional sector, with catering machines, we see more advantages in stainless steel kettles - but here the pressure and temperature stresses are also significantly greater than with "household machines".
