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The most common place for cooling flushes is in Heat exchanger machines where the water in the boiler is at 125C (or there abouts - depending on boiler pressure). After a short time at idle (several minutes) the water in the heat exchanger also gets to 125C.... which is just a tad hot for brewing coffee.....
So you push the brew switch and steaming, hissing water is released through the group..... and when the "water dance" stops - the water in the heat exchanger is the correct temperature for brewing....
Some non HX machines do need a cooling flush to reduce the temp of the boiler contents.... or to force the boiler to cycle so you can temp surf.... but more commonly they need a heating flush.... the water heats up the PF and group to the brewing temp before you load with grinds....
The procedure is very machine dependent.... so its a bit hard to provide a generic explanation....
As with all things, its probably best to let taste be your guide. Try out a whole bunch of flushing routines and see what tastes best!
One of the great things about the cooling flush is that once you have a little bit of experience, you can actually manipulate the flavour of the espresso a lot more quickly than you can with a PIDded machine because you dont have to wait for the system to stabilise after each change.
At home, if I have a blend that I want to extract cool, Ill flush water for five or six seconds immediately before locking and loading. For normal temp extractions, I just flush for one or two seconds, then lock and load. If I want to heat it up a bit more than that, I flush, then go and do the grind/dose/tamp thing and then go without flushing. That last one doesnt seem to make as much difference as the massive cooling flush, though.
Like Java said, machines vary in their performance. For example, some machines might require water to be flushed for six or even ten seconds to reach a normal temperature.
What I see is this: steaming and hissing and splattering (ouch) for a second or two,
then some entertaining jiggling around for about 5-7 seconds, then a fairly steady
stream.
I usually wait just until the steady stream starts, then lock and start the shot.
If I want to brew a little hotter I wait until not quite the end of the jiggle.
Im certainly impressed by the Makin Espresso machine I had a little tinker with on Saturday at First Pour. Even after quite a long idle time, I went to pull shots and the cooling flush was tiny!
Do you flush till you get the boiler to cycle or just a couple seconds so as to not get the boiler to cycle?
If the boiler does cycle when you do a cooling flush when you should lock pf in and start shot, right away, as soon as the boiler finishes cycling or wait X seconds after boiler cycle has completed?
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