article on improving HX flushing https://www.home-barista.com/hx-love-manage-brew-temperature.html
I'm usually pulling the shot within 30s of flushing, the idea, that if you pull it too early, there maybe a temperature hump. - hmm
The steps of the complete ritual including the "water dance" cooldown flush are as follows:
- Draw water through the group with the portafilter in place.
Continue until the stream pours steadily for about two ounces (more or less depending on your taste) since the group is still over temperature when the hissing stops. It depends on the machine's pressurestat setting, but in general the water temperature is around 206-207°F when the last of the sputtering subsides. You can literally count "one-one-thousand, two-one-thousand, three-one-thousand" to tick off each degree from that point. To more easily judge when the heat exchanger has emptied of superheated water, keep the portafilter loosely engaged and listen for the sounds of hissing steam and boiling water.
If you've pulled a shot within the last five minutes, you'll need to pay extra attention because the cooling flush will be much shorter than if the machine has been idle for 10-15 minutes (on the order of ¼ the amount of water). You may hear only a brief hiss and that's it. Don't draw too much water or the next extraction will be under temperature (sour).
.........
I've noticed two things the operator controls that affect the HX hump, especially on prosumer E61-type espresso machines—how much is flushed, and the length of the interval between flushing the water out of the HX and starting the extraction. The flush amount tends to affect the mid-to-tail end of the curve. The rebound time ("pause before the pull") affects the very early part of the shot, either producing a prominent hump, a flat one, or if there's little or no delay, it disappears completely and the curve becomes more like a rising straight line that barely reaches the desired peak temperature (or worse yet, forms an inverted U as the temperature plummets soon after the extraction begins).
The ideal rebound time for the E61s I've measured is somewhere around 15-35 seconds, depending on the boiler temperature. It is certainly less than one minute since the heat exchanger water is above 212°F at that point for all reasonable pressurestat settings.
I'm usually pulling the shot within 30s of flushing, the idea, that if you pull it too early, there maybe a temperature hump. - hmm