Cooling flush for HX machines
Many articles on Nth American sites talking about the need for this. Various comments on coffeesnobs saying this isnt required if the machine is configured well. In technical terms is someone able to articulate the rationale for not flushing a HX machine, ie what it means for it to be properly configured in a way that doesnt require a flush?
My understanding is that the water temp right after a shot is pulled on a good machine would likely be closer to the desired temp than if you waited a few hours with the machine turned on before making a 2nd shot, and that a cooling flush after that multi hour wait would bring the temp back to where it should be?
A cooling flush is only required if the circulation of hot water from the heat exchanger to the head while the machine is 'at rest' is too rapid, so that the group head becomes hot enough to cause the water being pumped through the heat exchanger to the head during extraction to overheat beyond the desired temperature, and in extreme cases getting hot enough to flash to steam as it exist the head. A cooling flush removes that excess heat, bringing the water temperature at the head down to where it should be.
On most heat exchanger machines the head is connected to the heat exchanger by one pipe from the top of the HX and another from the bottom. With the machine 'at rest', the water in the HX heats, rises to the top of the HX and travels slowly to the head, loses heat to the head casting, cools and travels slowly back down the other pipe to the bottom of the HX. This is the thermosyphon effect, which is what is used to head the head casting up to operating temperature. A machine that is 'configured well' is one where the operation of the thermosyphon is controlled to ensure that the temperature of water out of the head is the same regardless of whether an extraction has just occurred or the machine has sat unused for hours. It is most commonly done by fitting a restrictor to the upper HX pipe - often a small brass plug pushed into the boiler end of the upper HX pipe with a small hole drilled into it to reduce the water flow to slow the transfer of heat to the head.
Restrictors in the HX pipe can actually work too well - I had to fix a Rancilio Classe 8 once that actually required a warming flush before extraction, as the HX pipe fittings were almost completely blocked with scale, preventing the HX from thermosyphoning properly.
Thanks for the detailed response. Very impressive and exactly the sort of detail that I was looking for.
Couple of follow ups if i may:
1. This restrictor on the upper Hx pipe, is it something that the machine generally comes with as standard from the factory? I am thinking of some of the more common HX prosumer machines as made by Rocket, Vibiemme, ECM etc...
2. Are there a few brands that you consider to do a better job at staying temp stable over a period of time and not needing a coolng flush?