E61 Thermosyphon

How your E61 group stays hot

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The E61 Thermosyphon (the process that keeps the group hot) is quite simple, but perhaps not always clearly understood. It is, however, important to ensure your shot is extracted at the right temperature to make the perfect espresso. The diagram below shows where hot water enters/leaves the chamber at the top of the group. This hot water can be taken from high up on the boiler of a single boiler non heat exchanger machine, or directly from the top of the heat exchanger (HX) on an HX machine. The water returns to either a lower point of the boiler or heat exchanger and because it has given up some heat to the group sets up a convection current which starts water slowly circulating from boiler/hx to group and back.

Only the portion of the group shown with the dotted red oval actually has water circulating within it. The chamber at the top of the E61 group containing this water can be seen circled in red on the inset photo. The heat travels to the rest of the group by simple conduction through the brass of which the group is constructed. I think that there can be a mistaken impression that the space within the group where this hot water circulates is much larger than it actually is. Another point to remember, this chamber is far from empty, it also has the E61 "mushroom" assembly inside it!

The far point of the group (the bit you lock your portafilter into) reaches about 80C in an HX machine and will be a little cooler in a non HX machine. Any machine whether single boiler, HX, or twin boiler, will always have the group (where the portafilter locks in), at a lower temperature than the brew water. Companies like Dalla Corte, Marzocco, Synesso etc.., use a “saturated” brew group to try and overcome this effect.

“Saturated” brew group is a fancy term for having as much of the brew group (effectively the portion of the group nearest the portafilter) as possible filled with hot water, either by chambers within the group, or bolting the boiler on top of the part which holds the portafilter. These types of configuration inevitably limit the slight cooling effect of the cooler group (because the group is nearer brew temperature) and increase temperature stability during the shot.

Note: the temperature of the E61 group can be further managed by the use of various “thermosyphon restrictors”, really little plastic washers with different sized holes in them placed in the pipes leading to and from the group.

(click to enlarge)