Espresso! My Espresso!
Understanding and Preventing Thermosyphon Stall
by Randy Glass - Copyright 2009 (updated 2020) - All rights reserved
E-mail me at email@example.com
NOTE: While this article was originally written to address Heat-Exchanger systems ("HX") it also applies to double boiler machines. Here is a simple diagram of a double-boiler machine's brew boiler. As you will see, there are only two major physical differences:
1) The double boiler has no internal tubing to heat the brew water. That is the heat exchanger tube heat exchanger tubing.
2) The double boiler is completely filled with water instead of storing the seam above a reservoir of hot water.
In terms of this article, the rest of this article applies to both designs.
An espresso machine with a heat exchanger system and a thermosyphon system is a great way to get a high performance espresso machine in a relatively small space. It allows the user to pull an espresso shot while at the very same time steam a pitcher of milk, and the amount of heat energy stored in a boiler allows most users to brew espresso as quickly as possible. But there is no free lunch, and there are problems with such a system.
Another possible cause of thermosyphon stall can come for a leaky upper chamber seal in the E-61 brewhead. If the chamber seal leaks slowly, the escaping water may not be seen by the user as the heat of the group can vaporize the drips before they noticeably reach the drip tray or portafilter. The source of the water is from the heat exchanger and since it is not replaced by the autofill system of the machine, if this leakage goes on long enough while the machine is idling, it can form an air pocket in the thermosyphon path and the result will be a stall. There may be other causes of water loss in the thermosyphon path, but whatever the cause of the water loss, the end result will be the same if the "bubble" is large enough to stop the convective circulation of water.
To prevent this, whenever doing a cooling or cleaning flush, avoid doing short flushes. A longer flush pushes out any steam or air, and it will replace enough of the hot water in the thermosyphon system with cold to lower the temperature to below the boiling point so that the flash boil cannot take place. Whenever opening the group to the atmosphere when the machine is at or near operating temperature, flush for at least a few seconds. I do not know what that amount of time would be, but generally, the longer the machine has been idle, the longer the flush. I would guess at least three or four seconds. I have eliminated stalls since conscientiously avoiding short flushes.
If a stall takes place the only solution is a very long flush to rid the system of air. In severe cases this might have to be repeated three or four times with a bit of a rest between flushes to reinstate the thermosyphon flow.