Finally, the service manager agreed to check it out with the understanding that if I was wrong, I still had to pay for the time they spent investigating.
I was right :-)
I once moved into a place that was great, except the shower would start out hot, quickly drop to warm, and then stay warm for as long as we cared to try it. I hated this, so every time I showered I'd spend some time trying to debug it, despite knowing nothing about plumbing.
Eventually I asked myself: why do normal hot water heaters work the other way, where they stay hot a long time and then get pretty cold? Clearly, there's some way of keeping the incoming cold water separate from the already-heated hot water, like by putting the output pipe at the top and the input pipe at the bottom. But if you reverse them, you might get what we had.
I leapt out of the shower and felt the hot water heater pipes and sure enough, reversed. My landlord came over shortly thereafter and fixed it, and I felt very smug when I had my first properly hot shower.
Turned out the installer had failed to put in the high altitude kit which told it to expect a smaller pressure differential. Mostly it was fine, but on rainy days with low barometric pressure, the difference would drop below threshold. But I had to be home for enough rainy days to figure it out.
That's a great quote. I think I'll use that instead of "where there's smoke ..." from now on.
Bought my own solenoid online and surprise the bike started working again.