Mystery Solved

So when we were in California pouring over all the "bits" that came with Kamalii, I found myself pondering a couple of very large metallic rods with screw ends, that looked a lot like gigantic engine cooler anodes, but which were way too big and did not seem to be made of zinc. They were filed in the "No idea, but I had better keep hold of them" bin.

Cut to last week, when going through my file of documentation I stumbled upon the installation manual for the hot water cylinder, and discover it is fitted with a magnesium anode, which should be checked every year. I figure it had probably only been about 15 years since it was last checked...

Bingo. Looks to me like it was on it's last legs....

Old (left), and new (right),