What is the number one killer of espresso machines?

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That's lime, that much was clear.

...the number 2 killer of machines is - descaling!

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This is no joke; descaling is the biggest stress on machines, and almost every descaling process results in something breaking or starting to break. This is because dissolved particles always remain in the machine, which later clog or damage something. How do you break this cycle? From day one, use only water with a hardness of less than 8°dH.