Espresso! My Espresso!
Understanding and Preventing Thermosyphon Stall
by Randy Glass - Copyright 2009 - All rights reserved
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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 one is the 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 turning on 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 is addressing- thermosyphon stall.
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.