Espresso! My Espresso!
Understanding and Preventing Thermosyphon Stall
by Randy Glass - Copyright 2009 (updated 2020) - All rights reserved

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.
      A minor problem is overheating of the brewgroup. The circulating hot water can cause the user to brew espresso at too high of a temperature. That is easily remedied by engaging the brew function and running an amount of water through the group into the drip tray to cool the system before pulling the first shot in a session. Once again- no free lunch. That cooling flush can lead to another difficulty with thermosyphon systems, and this is what this article addresses- thermosyphon stall.

      The diagram above represents a thermosyphon system. It operates on the principal of convection. Water in the heat exchanger tube is heated by conduction from the hot water and steam in the boiler. Heated water becomes less dense and so it is just a bit lighter and it rises (heat rises- think hot air balloon). The hot water rises and goes to the top of the heat exchanger and into the upper thermosyphon pipe. It gets to the brewhead and there it transfers its heat energy to that mass of meta which heats the group. The water, losing its heat energy, cools and becomes a bit heavier and "sinks." This cooler water moves downward into the lower thermosyphon pipe and moves back to the heat exchanger. As this convection current continues the water moves in this circular path, picking up more heat energy each time it passes through the heat exchanger and losing it when it reaches the brewgroup. This flow is indicated by the white arrows.
      This is all well and good, but as the machine reaches its ultimate temperature the water in the thermosyphon path becomes quite hot and is under pressure. It nears the boiling point, and in some areas, because the system is sealed, it can actually achieve or even exceed the boiling point of water slightly. That's fine as long as the system stays sealed, but when you are getting ready to pull a shot, what do you do? You unseal the system, opening it to the atmosphere as illustrated above. The pressure in the heat exchanger system suddenly drops as the hot water has an escape route through the brewgroup. Since water at a lower pressure boils at a lower temperature, there is a potential for a small amount of water in the thermosyphon to flash boil as the pressure is released and a pocket of air and/or steam can form in the thermosyphon.
      As shown here, that pocket of air/steam rises to the top of the system (if it isn't already there), and as it heats it will expand causing a pressure dam that will completely stop the thermosyphon circulation and the group will rapidly cool off. How do you know if you have a stall? The brewhead will be noticeably cooler by as much as thirty or forty degrees lower than normal. This will result in a very cool espresso extraction that will taste quite poor indeed.
      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.