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Bezzera BZ99 cooling flush and pressure gauge

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  • Bezzera BZ99 cooling flush and pressure gauge

    So while I was searching the forum for advice on how to resolve my issue with water flow on my Bezzera BZ99 (now solved with a descale) I’ve stumbled across a number of posts about how important is the cooling flush on an HX machine to stabilise brew temperature.

    I would like some advice on what is a good starting point as length of the flush on the BZ99, and what is the method for tuning the flushing time for optimal brew temperature.

    Also I was wondering if anyone has installed a pressure gauge on the BZ99, if there are kits available, and if it’s worth doing.
    TIA

  • #2
    Apparently if the BZ99 is set up correctly it shouldn't need a cooling flush, however being an Italian machine I understand that they are typically set up for Robusta beans which require slightly higher temperatures.

    As for the cooling flush, all it requires is visual observation. Based on the concept of 'Boyle's Law', there is a relationship between temperature, pressure & volume. Like a pressure cooker, the boiler effectively has a higher pressure than ambient conditions, which results in a higher boiling point for the water inside. Therefore, when you run the group head the water sitting in the heat exchanger is heated to a level above 100 degrees C and as soon as it exits the group head and into ambient conditions it spontaneously boils - the main issue here is that water being delivered to a filled portafilter is too hot and burns the coffee.

    To perform a 'cooling flush', once you've filled & tamped your portafilter but before attaching to the group head, hit the green button and watch the water coming out of the shower screen. You'll likely see the water initially bubble an fizzle as it exits the shower screen, and then settle down into a more natural flow. At that point the super-heated water has been flushed and cold water from the reservoir is being drawn through the heat exchanger. I'm not sure about timing but you'll want to attach the portafilter and extract without too much delay as the stagnant water sitting in the heat exchanger will start to heat up again.

    Others on here will probably have a much better idea of the science & machine characteristics, however as a fellow BZ99 owner this process is part of my workflow and produces results my family is happy with. Make sure to let us know how you go!

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