Finally got around to stripping down the offending unit:

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The element assembly looks remarkably clean for this area (very hard water), but I know the shower has only seen occasional use.

I can't see anything obviously wrong, no excessive scale, the elements are showing 13.1 and 13.6 ohms, and even on the 1kV range of my megger I'm reading insulation resistances in the hundreds of megohms range.

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