> "controllers" being less than perfect, I guess many installations have a
 > ventilated header tank so that the pressure is incapable of rising
 > excessively, even with max heating maintained.
Absolutely. There's always an expansion tank with a pressure regulator and 
relief valve. The "boilers" will often incorporate a fusable plug in the 
firebox. This is a plug with a lead alloy in it that is screwed into the 
water jacket. Everything is cool as long as there's water in it, but if it 
goes dry, the lead plug melts, allowing steam to enter the firebox and 
extinguish the flames. I'm not sure if home-heating systems (especially hot 
water, as opposed to steam) have these, but they're mandatory in larger steam 
boilers, even the old traction engines.
 > I know that in northern Europe, some thermal power plants sell piped 
 > steam, metered at each home, for District heating.
I think they have meters on the condensate return to tell how much steam is 
consumed. The one building that I maintained had such meters for the 
different floors, presumably so the various departments could be billed for 
their share of the heat. They had been abandoned many years ago, but still 
hung on the wall in the basement.
And of course, many college campuses are heated by central steam plants. When 
I was attending Ohio State University, it was considered great sport to go 
'tunnel stomping', exploring the extensive tunnel system beneath the campus. 
The tunnels ranged from modern, large, and well-lighted, down to ones that 
you had to crawl on your belly to get through. (Or so I heard, heh, heh)
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