Cooling flush? Nope - it's a heating flush

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#1: Post by BaristaMcBob »

My Bezzera BZ10 is an HX machine, but unlike the E61 which circulates boiler water through the group, this model has electric heating elements in the brew group.

Conventional wisdom says to perform a cooling flush on HX machines. However, I've been adjusting the pressurestat and measuring brew temps and realized that this machine requires a flush - but not because the water is too hot...but because the water is too cold! This is opposite of what I expected.

Initial brew water temp from an idle machine: 190F
Then a 3 oz flush
Post-flush temp: 203F

Anyone else observe this with Bezzera BZ style machines?

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#2: Post by HB »

A more accurate term would probably be "thermal stabilization flush". Some more background:

For all the HX espresso machines I've measured, the grouphead idled below the target temperature. So the "cooling flush" warms the grouphead and cools the heat exchanger. How much the grouphead needs to be warmed up (or the heat exchanger cooled down) varies depending on the design: Ideal brew temperature management by HX espresso machine type presents my preferred nomenclature.

As if that weren't confusing enough, for many heat exchanger espresso machines, the grouphead will need time to cool down under load. The grouphead acts as a thermal stabilizer; under load, it will accumulate heat from the prior shots and no longer be below the target brew temperature. Recognizing all these factors may confuse home baristas, many manufacturers have added thermosyphon restrictors to their HX designs. Using my nomenclature, they're very strong "Mixer" types, so they need little to no thermal stabilization flush if they've been idle for a few minutes. That makes for less complicated temperature management for most home baristas who only pull one or two shots; the downside is that their brew temperatures drop under heavy load (e.g., they'll need 90+ seconds between extractions).

This video below elaborates on these points.
Dan Kehn

BaristaMcBob (original poster)

#3: Post by BaristaMcBob (original poster) »

Very informative! Thank you.

Gosh - is there no end to the depth of espresso knowledge :D