Of course, just to make a liar out of me, it seems ok now after I went and tested it. It's mocking me! (heh)
However, that just further confounds the situation. Why would it be lukewarm for the most part, once in a while normal/hot, and the sinks always hot?
Also, I'm told that the tenant downstairs needs to clean "sludge" out of the heater every few days (this is her excuse when I tell her that the water is cold, "oh he probably hasn't cleaned/drained the heater"), which . . .sounds weird to me. I asked an independent plumber though, who'd seen the thing, and he says it's new and one of the best.