In this video, BH Art Director, Jacob Thue shows us the inner workings of the saturated group head on the Kees Van Der Westen, Speedster.
In 1970, La Marzocco released their GS machine, which featured the group head welded directly onto a smaller brew boiler. The system was totally filled with water, hence GS for gruppo saturo (Italian for ‘saturated group head’). La Marzocco is well known for fabricating its boilers out of stainless steel rather than brass. They commenced this practice with the release of the GS machine. Although the boiler was made of stainless steel, the GS machine’s group head was crafted from brass.
As you can see in the illustration above, the group head is hollow, with a large reservoir designed to be fully saturated with water. The group is welded directly onto the steel boiler. Through the force of convection, the hottest water in the boiler rises to the high point at the group head. The brass group radiates heat into the outside air and then the cooler water in the group syphons down to the boiler, where it is reheated (similar to the way the E61 thermosyphon worked). Early iterations of this machine were not insulated and therefore required a large offset between the temperature in the boiler and the exit temperature of the brew water.
More recently, La Marzocco has wrapped their brew boilers in heavy foam insulation to reduce the offset temperature and reduce energy expenditure. This allows the water in the brew boiler to be maintained at a temperature only a few degrees hotter than its exit temperature.