The Mirage`s heating system is based on the so-called thermo-syphon system. It consists of one large copper boiler (13ltr for the Duette, 19ltr for the Triplette), equipped with a heat-exchanger (HX) for each group, running slanted across the boiler.

This HX is connected to the rear of the group by a thick tube running from top of HX to group and another tube running from rear of group to bottom of HX, completing a circuit.

The water in the heat exchanger is rapidly heated by the surrounding hot water and steam within the large boiler. This method is extremely efficient, as it utilizes the high energy stored in 125°C steam and the excellent heat-conducting properties of copper. This makes it significantly more effective than a traditional heating element. Due to this efficiency, the HX can be relatively compact; typically, a 450cc HX is used per group.

Hot water naturally rises because it becomes less dense when heated. As the water in the HX heats up, it rises through the upper thick tube connected to the top of the HX and flows toward the group head. The group head sits slightly higher than the HX and is a solid 4 kg piece of brass, known for its high thermal conductivity. As the hot water enters the 35cc chamber at the rear of the group, it transfers its heat to the brass, which heats up the group.

Once cooled, the water becomes denser and flows downward through a lower thick tube back into the bottom of the HX, completing a continuous cycle. This process is called a thermo-syphon loop and requires no pump or mechanical assistance—it operates solely on thermal dynamics. Even when idle, this loop circulates constantly.

Because heating water in a sealed system increases pressure, an expansion valve is installed. It’s set to release small amounts of water at 11–12 bar, while the group solenoid valve, rated at 15 bar, acts as a backup safety measure.

The group sitting at the very end of the thermo-syphon loop is a vitally important stabilizing factor. It is after all the last part water travels through before hitting the coffee, during brewing. Its temperature therefore is important and should be very close to the desired temperature of the brewing water.
The temperature of the group body can be adjusted simply by having more, or less hot water flow through its rear chamber. The temperature inside the HX being quite a bit higher than the desired temperature for the group, the thermo-syphon loop needs to slow down, needs to be restricted.
With the Mirage we adjust the boiler to 125 degrees Celsius. To get the group body temperature at low nineties we install a standard restrictor at the top of the HX with a passage of 1.5mm.

With different restrictors installed it is quite possible to operate one machine with temperatures adjusted differently from group to group, for example to accommodate for the use of lighter and darker roasted coffees. As an option we offer adjustable restrictors.

One also may attempt to change the group temperature by changing the boiler temperature, but this method is limited because large boiler temperature changes are needed, to realize just a little effect at the group. Also, the steam or hot water capacity of the boiler may be affected that way. With the proper restrictor one can for example easily combine the extremes of 1.5 bar high steam power with a low 88 degrees C. brew temperature.

The challenge of the thermo-syphon system lies in delivering brew water at a stable temperature in the low 90s°C, even though it comes from a heat exchanger (HX) storing water at around 125°C. This high temperature is necessary to keep the group head at the ideal brewing temperature, while the water itself must be cooled slightly before reaching the coffee. The brewing process begins when the group solenoid valve opens, allowing water from the thermo-syphon loop to flow toward the coffee in the portafilter. Simultaneously, the pump activates, ensuring a steady 9-bar pressure from pump to coffee.

As soon as the solenoid valve opens, the flow in the lower thermo-syphon tube reverses direction—now moving upward to the group instead of downward to the HX. The upper tube’s flow continues as before. This is where the restrictor plays a vital role. With a 1.5mm orifice in the upper tube, its flow is heavily limited. The lower tube, however, has a much wider 8mm diameter and no restrictor, so about 99% of the brew water reaches the group through this lower path.

This is ideal, since the lower tube contains cooler water (around 88–92°C) compared to the upper tube (115–125°C). In the Mirage, we optimize this further by making the lower tube extralong, holding 30cc of water. Some remaining water still comes directly from the HX, which is constantly refreshed with cold water entering from the bottom. As hot water exits the HX through the lower outlet, incoming cold water flows alongside it through a narrow copper tube. This crossflow effect helps cool the hot water and pre-warm the incoming cold water. The fresh cold water is then forced to the top of the HX via an injection tube, and during brewing it travels back down to be reheated—ensuring efficient use of the HX’s heat.

Though this cooling process also depends on factors like climate and season, it reliably reduces water temperature well below 100°C. As the water travels through the long lower tube, its temperature stabilizes further. Finally, it enters the large 4kg brass group, where the thermal mass helps regulate temperature perfectly.

To improve this, the internal design of the group maximizes water contact with the brass. Water first flows into a 35cc rear chamber, then to the solenoid, then back into the group, filling the infusion chamber, and finally passes through the dispersion screen.
Thanks to the group’s thermal stability, even after boiler refills, many shots can be pulled at the correct temperature. For best results, a short flush (30–45cc) is recommended before brewing. Avoid longer flushes, as they destabilize the system, requiring up to five minutes to recover.