While a few years ago high-quality espresso machines used copper or brass boilers, recently there has been an increasing number of suppliers 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 boilers - proven for many years
Compared to stainless steel, copper has a very high thermal conductivity . This means that copper conducts heat very well. For this reason, copper has always been used for the highest quality cookware. When combined with water, a copper kettle relatively quickly forms a protective layer of copper oxide, a water-insoluble layer that protects the copper and has a completely neutral flavor.
Copper kettles should not be descaled with strong acids to avoid damaging the protective coating. Copper, as a mineral, is a very important trace element required by the human body. Our modern water pipes are also largely made of copper. Traditionally, copper kettles were preferred for thermal and mechanical reasons—it's no coincidence that the founders of modern coffee machines, such as Giuseppe Cimbali, were coppersmiths. To make copper even more resistant, it is now usually coated on the inside. Mineral deposits such as limescale have a harder time building up and impairing the machine's function.
Portafilter machines with stainless steel boiler - the newcomer
Stainless steel is both thermally and chemically very inert and therefore resistant to mechanical and many chemical influences. Stainless steel is tasteless and very pressure-resistant. Modern high-performance machines therefore increasingly feature stainless steel boilers. However, stainless steel is significantly more difficult to process, as 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 its heat output, measured by volume, is often higher than that of comparable copper boilers.
Copper kettle vs. stainless steel kettle - which is better?
Stainless steel is particularly effective in large-volume boilers. Here, the heater (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 water constituents 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 boilers with heat exchangers, which heat the fresh, oxygen-rich water for coffee preparation. Here, copper offers shorter paths and faster reaching of the target temperature, while simultaneously reducing energy consumption.
However, in modern machines controlled by temperature sensors (e.g., portafilters with PID systems), the boiler temperature, among other things, is measured directly at the boiler. Here, too, the heat-conducting copper boilers offer an advantage. Even slight temperature fluctuations are transmitted via the copper boiler to the thermostat, allowing the heating system to quickly and sensitively adjust the temperature.
Boiler repair
Boilers are particularly vulnerable to wear and tear at the points where they connect to the heating system, the valves, and the heat exchangers. If repairs are needed after many years of operation, copper boilers are easier to repair than stainless steel boilers. Leaks in copper can be soldered, and custom pipe fittings can also be quickly completed (in our specialist workshop). Stainless steel boilers that develop leaks are usually completely replaced or must be sent back to the manufacturer for repair.
Conclusion on the boilers
Both materials are very suitable as boiler materials. Copper absorbs heat very well and transfers heat to the water more quickly. This is a particular advantage 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 clean, which also applies to specially coated copper boilers. Copper is easier to process than stainless steel, but the pure material value of copper is higher. Therefore, the boiler material generally makes no difference in price.
For us, copper kettles are still the first choice in the semi-professional sector. Stainless steel is equally good in terms of basic suitability and quality. The deciding factor is whether you like the machine and its operation. In the professional sector, for catering machines, we see more advantages in stainless steel kettles – however, the pressure and temperature stresses are also significantly greater than with "home" machines.