The isolated steam boiler is located at the rear of the machine and has a total capacity of 10.5 litres. (Duette and Triplette, about 75% filled with water). An electronic temperature controller gets its information through a probe located above the water level and activates the heating element via 3 solid state relays. The heating element provides a powerful total of 4.800 Watt (normal) or 6.000 Watt (high power). This is divided over 3 heating spirals mounted on one flange. There are no moving parts in the complete heating system. Each heating spiral is connected to a control light at the bottom of machine. Combined with the machine`s display this enables for a quick visual control to check from the outside of the machine whether all parts in the heating system function as intended.

The steam boiler is made in stainless steel 316L, which is more resistant against aggressive water than stainless steel 304 and has a long life expectancy in an espresso machine.
The controller offers a programmable ECO mode, lowering the temperature of the steam boiler to about 70 °C during longer periods of idle or overnight.

Steaming
As standard, the steam valves have a turning knob. From closed to fully open requires less than half a turn. As an option, the mechanical steam valves at left and/or right can be activated with a toggle type lever or even by foot activation: an air bulb on the floor activates an electrical switch inside the machine which in turn activates a solenoid valve with a large passage. This solenoid valve is either fully open or fully closed. The steam flow/power can be decreased by the use of the original steam valve to match the size of the pitcher. When adjusted, the foot-activated valve opens always immediately at the preferred flow, enabling speedy steaming.

Hot water dispensing
Hot water can be dispensed in two manners:
1. Pressing the push button switch results in dispensing mixed water for an adjustable number of seconds. The switch activates two solenoid valves and the pump simultaneously: hot water from the steam boiler passes through one valve, the other valve supplies cold water at pump pressure. Cold water addition is restricted by an adjustable needle valve in the manifold such that the temperature of the water dispensed can be adjusted between approximately 60 and 100 °C. This mix of hot and a bit of cold water results in a steady, non-spattering stream from the hot water spout and can be adjusted to be excellent for making Americanos or green tea.
2. When the toggle switch is pressed downwards only the hot water valve opens which results in water flowing directly from the steam boiler to the spout. Releasing the toggle switch will stop the flow immediately. At the moment the toggle switch is pressed down the temperature of the water dispensed quickly rises to boiling point and will start to sputter. The boiling water is excellent for making black tea.

Steam boiler fill
Filling of the steam boiler is prohibited while a group is active. This setting eliminates flow-rate decrease and subsequent variations in brew-pressure during the infusion phase. The disadvantage of this setting is that the steam boiler may never start to top-up in a high-volume location where at least one of the groups is active for uninterrupted periods of time while also dispensing hot water and, with minor impact, steaming milk. For extremely high-volume locations the setting can be changed such that the steam boiler starts filling as soon as the water level drops below operational.

At first installation it is important all air is bled from the coffee boiler. Air compresses; water does not. When you first activate the group solenoid and pump, incoming water expels air because the pickup tube sits directly under the group top, the system’s highest point. Once the system reaches its target temperature, the challenge is avoiding temperature variation before water reaches the coffee.
Thermosyphon flow begins with temperature change: heated water rises, flows to the top of the group head, cools slightly, then returns to the boiler bottom. Boiler and group geometry allow warm and cool flows to pass freely. The most stable temperature, called the “sweet spot”, forms under the group top, where the pickup tube sits, feeding the stainless steel three-way solenoid valve at the boiler’s lower, hottest location.
During brewing, water never leaves the boiler/group assembly. It runs through a tube under the group top to the solenoid valve, then through another tube to the group bottom. Both tubes run parallel high in the boiler, adding thermal stability. The stainless group bottom plate sits low and remains cooler than the water; the system prevents heat loss here.

The dispersion block extends through the bottom plate so water enters it without touching the metal. Made of temperature-neutral polymer, it stays cool, preventing coffee oils from baking on and keeping it clean longer. Two block sections ensure even dispersion.
The group screen is held by the gasket. A supplied tool removes the gasket and screen for service. The gasket seals against a small ridge in the group bottom; its flexibility provides soft locking without force and resists heat-induced hardening.

The screen’s circumference seals against the group bottom, blocking grounds from entering the group. Its thick base resists deformation, is perforated, and covered by a stainless membrane for even extraction and effective filtering during pressure release, helping keep the group interior clean.

The Slim Jim is equipped with multiple boilers: one large stainless-steel steam boiler to produce hot water and steam, one stainless steel boiler per group to brew coffee. A heat-exchanger with balanced cross-flow and an electronically controlled cooling circuit ensures well-conditioned supply of pre-heated water for the coffee boilers to ensure enormous capacity in a relatively compact machine. Each boiler has its own electronic temperature control system, heating element and safety devices, offering extremely precise temperature stability at high volume.

The importance of pre-infusion still seems to remain under-appreciated. On machines offering a very confined brew temperature a well-executed pre-infusion becomes imperative: without they will not be able to extract the full range of complexities hidden in the coffee. Properly infusing the grounds makes for uniform wetting and swelling of the grounds which in turn reduces the danger of channelling while also locking in fines, thus preventing them from travelling downwards and clogging the filter.

Even more importantly however, pre-infusion allows a tranquil warming and softening of the fats and oils in the coffee grounds before extracting these with high pressure. This clearly results in a higher yield of solids in the cup. These fats and oils are the actual carriers of the delicate aromas and flavours. A good infusion system allows for a finer grind with larger surface area making it easier to wash out the oils and fats.

When the brew process is started, the group solenoid valve is activated and the pump starts to run. A 0.6 mm restrictor is located at the supply side of this 3-way valve reducing the maximum flow-rate through the valve. After passing the group-valve the tubing splits in two: one line feeds water onto the coffee-bed, another line feeds water into the pre-infusion cylinder.

Movement of the piston of the pre-infusion cylinder is restrained by a double spring gradually increasing load while being compressed. At the same time pressure onto the coffee bed gradually increases. At about 4.5 bar the piston has reached it total travel and from that moment on brew pressure rises quickly until pump-pressure is reached. The pressure gauge is connected to the pre-infusion cylinder which means that there is an open connection all the way to the coffee puck. The gauge displays the actual extraction pressure, not just the pump pressure. The barista can monitor exactly how infusion and extraction pressure develops.

When the group-valve is de-activated at the end of a brew, pressure on the coffee bed is immediately released as flow in the valve reverses and water in the pre-infusion cylinder is pushed through the group-valve into the machine’s drain thus flushing possible very fine coffee grinds out of the valve after each brew.

The steam boiler has two probes to monitor the level of the water inside. The lower probe ends just above the heating element. When the machine is fired and filled for the first time, the brain only switches on all heating elements, when the lower probe detects water. During filling up, the coffee boilers fill quicker than the steam boiler because filling starts immediately when the water connection is opened and the coffee boilers are smaller. When the lower probe in the steam boiler detects water the elements inside the coffee boilers are already submerged. This system ensures the elements cannot be turned on while not being cooled by surrounding water.
If the steam boiler level would ever fall under the safety probes tip, the machine shuts down completely with the led at group lever or touch pad blinking as an alarm. This alarm mode is also activated when one of the group solenoids or the filling solenoid is activated continuously for longer than 2 minutes. The machine can be re-set by switching it off and on again.

When the higher probe contacts water the brain de-activates the fill solenoid and pump. These are activated as soon as the higher probe does not detect water. All temperature settings have a restricted upper temperature programmed. When this temperature is reached no more signal to the solid-state relay is sent. Each boiler is equipped with an overheating cut-out switch. When this switch detects too high a temperature, it will disconnect power to the element. The steam boiler is equipped with a safety valve. The coffee system, including the heat-exchanger, is equipped with an expansion valve.
The group solenoid valve may act as a second expansion valve, rated at 15 bar, in the event the original expansion valve would fail.