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The influence of flushing prior to extraction

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  • The influence of flushing prior to extraction

    Hi Snobs,

    Just like to enquire whether any folk have experimented with different amounts of time flushing the group prior to putting the portafilter in and starting the extraction. If so, what have you found?

    Clearly a variable confounding generalisations is the type of machine used, but Im keen to investigate the influence in a little more detail.

    For e.g. would more flushing enable hotter water at the start of the extraction on a machine without pre-infusion? Does flushing have a significat impact on machines with pre-infusion?... there are many a questions here, hope you get the idea.


  • #2
    Re: The influence of flushing prior to extraction

    You would flush prior to extraction on a HX machine that has been sitting idle for some time. Often times the water temperature is way too high and will burn the coffee. Youll flush until you hear the water stop "spitting."


    • #3
      Re: The influence of flushing prior to extraction

      There is no answer due to the variabilities of machines

      If your talking about a cooling flush on a HX machine then what WOTB wrote is correct. But for different styles of machines a flush can be used for other things.

      My 3 group if left sitting idle for a fair while outdoors benifits from a heating flush to raise the group and PF temperature up due to airflow going past cools the large surface area of the exposed groups. If its in regular use a bit like cooling flushes on a HX its not really needed. I really need a custom Scace type PF to confirm whats happening but it works to my taste and feel.


      • #4
        Re: The influence of flushing prior to extraction

        Originally posted by 2027232C242E3B2B2C25420 link=1271487971/2#2 date=1271496260
        But for different styles of machines a flush can be used for other things.
        Flushing is also used for getting rid of the grinds left on the shower screen by the last shot.


        • #5
          Re: The influence of flushing prior to extraction

          Flushing is certainly a way to keep the group clean. In terms of brew temperature, I think that theres more to it once you start scratching the surface ...

          Some of the more sensible thinking that I have read about the e61 heat exchanger flush is this: you simultaneously purge overheated water from the heat exchanger whilst warming up the e61 head a little. Putting it another way, the HX in some machines might run hot, but the e61 is pretty much always going to run cold. This sounds fairly plausible to me. It accords with the scace thermofilter measurements that show that shots from e61 machines tend to heat up a degree or so per shot across the first three or so shots in a series of ten, as well as the isothermal diagrams of e61 groups that show that the temperature drops off as you move down from the bolt on top of the group, past the m6 screw and towards the tip of the group. (It also underlines part of why I hate machine advertising based on how heavy the group is; have a think about it!)

          The simultaneous heating of the group and flushing of the heat exchanger also explains why there can be such different results from flushing; it is always going to depend on the heat exchanger - idle time, volume, placement, flow, etc. One of the fixes to get HXs to perform better and be easier to use is to restrict the flow through the HX, basically by inserting washers of a certain diameter at certain points in the flow. Getting the balance right is tricky and requires a bit of work. The iritating thing with it is that you cant tell from the outside of the machine if this has been done properly and, if so, what temperature the machine will idle at. Its certainly irritating to look at a$3000 prosumer machine that might be horrible for the sake of $0.50 of teflon. Imagine how competition baristas feel looking at a Nuova Simonelli Aurelia and wondering how it has been set up!

          With multiple boiler machines, the flush is always a cleaning and warming flush. The flush that you need will vary from machine to machine. I have seen some dual boiler machines that have a "chunk of metal bolted on to the boiler" type group and some with e61 heads; one expects that these will also pick up temperature across the first few shots of a series. The flush required for a saturated group multiple boiler machine isnt much, if anything. This is one of the reasons why they tend to be well-regarded in the industry. With a FB80 EE, for example, you barely need to flush. The exception is the volumetric La Marzocco Linea (the AV), which takes the water out of the saturated group head, through a copper pipe, through a flow meter, back through a copper pipe and back through the group. The tubing to and from the flowmeter cools down, so you need to flush a bit to heat it up. This problem was basically eliminated in the GB5/FB80 series with the "Pierro" group cap, which contains an integrated flowmeter.

          "Pre-infusion" is one of those irritating terms that is used differently to mean different things. If were talking about a machine with a pre-infusion chamber that sort of hangs of separately to the group, one presumes that it would present the same sorts of challenges as a flowmeter on a linea. If were talking about a machine that uses a washer to restrict the flow of water, then there isnt really an extra part to heat up, but I wonder how the interaction between flow rate and brew temperature plays out. Presumably - and Im just going off the top of my head - the slower flow of water whilst flushing gives more time for the heat from that water to heat up the group per unit volume of water, so greater heating effect for less volume of water ... I dont know.

          All of that aside, I think that flushing has one important function - to make the coffee taste better. If you are flushing and it isnt making your coffee taste better (or cleaning your machine), you are doing it wrong. There are lots of other inconsequential things in espresso preparation that you could be obsessing over. Or you could just focus on getting something that tastes better.



          • #6
            Re: The influence of flushing prior to extraction


            Another fantastic post Luca. And dont we all wish we could afford a saturated group machine.

            I only learnt about these restrictors through Dennis and his explanation one day of controlling temp on his Faema. It is fantastic gaining a greater understanding of the machine, but as you said, its whats in the cup that counts.