On Monday, 19 August 2019 23:11:57 UTC+1, NY wrote:
When I took the immersion heater box spanner back to the hire shop, the guy
told me about a customer who had forgotten a) to let the tank cool down, and
b) to empty it of all the water

I had a slightly similar experience; we (plumber and I) drained the tank down till the water stopped flowing out the drain valve, then loosened the pipes to walk the tank round till the bottom connection was at the front to tilt it to get the bottom water out.

I wondered aloud why the hole was so furred up in a soft water area and the plumber poked his screwdriver into it. At this point we realised that the tank was not empty and all that was holding the water in was probably a stray bit of foam insulation (which stopped holding the water in).

Fortunately there were two of us, and buckets handy. If I'd been on my own I'd have been stuck with my thumb in the hole until the postman called ... some days later ... don't get many letters these days.

Even the plumber was a little bit startled by the experience.

Owain