while the riviera steeps in its second vinegar bath, and i await parts, i weighed in on a robust discussion about flushing the e61 groupheads on heat exchanger machines like mine. (for the neophytes, basically the brew group on such machines contains constantly circulating water which, over time, grows too hot for brewing because of the necessity of maintaing a certain boiler tank temperature for simultaneous brewing and steaming. the solution: flush the group a bit to bring down the temp before pulling your shot.)
people stress about this. they want to know exACTly how much water to flush, what the seconds and ounces are, and how it should feel on your pinky finger. this goes against the indefinably sensory nature of the craft. such a routine should be both (a) second nature, and (b) flexible for the time of day, machine operating temperature and desired flavor profile. to plagiarize a rant:
measuring the water or timing the flush isn’t reliable, especially if you turn your machine off every day. the isomacs particularly can run very hot (meaning the temp takes longer to bring down, rebounds quicker and cycles through the “green zone” slower) or it can run somewhat cooler, within the first two hours of operation for example (meaning the group temp is slower to rebound and stays in the “green zone” for less time).
you listen first, getting a feel for when the hissing stops, plus a few seconds. eventually, you can watch as well, making it possible to flush precisely even with the grinder on. then you let the group temp rebound. this time can vary, based on (a) the kind of flavor profile you’re shooting for, and (b) the way the machine is behaving. making this a science just kills the finer points of nailing that superb pull. i’ll notice, for example, that while the machine is rebounding (for me, it’s typically two trips through the green light, then brew on the third) it may linger longer in the green zone. that tells me the machine is especially hot, allowing the heater to stay off for a longer period of time, and i’ll cut the rebound short by a few seconds before locking and pulling.
similarly, if i’m brewing a lot of shots and keeping the grouphead fairly cool with constant use, then my flushes are shorter and my rebound periods slightly longer. but the behavior of the boiler gauge, the sound of your flushes and of course the taste of your shots all help you get a feel for this. after awhile, it becomes unconscious. you can flush while grinding, then watch the green light cycles while tamping, then lock and pull a bit early or late, or even adjust the routine based on what kind of flavors you want out of a particular bean.
all this is a part of the e61 fun. stressing over flushes and rebound times only forces you into a set of rigid rules that should be flexible. my advice: taste a lot of espresso. get to know your machine. then tweak the factors based on your sensory feel and brewing whim. it takes time, but it’s not hard.
no reason to spend screedy breath over at CG without recouping a blogged two-fer! dan kehn, of course, owns the definitive HX-flush analysis, to which i have referred for grounding numerous times. the heat exchanger tuning chart, meanwhile, i found to be overkill.