>> "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.
 
Dump steam or condensate into a fired boiler?!!!
 
I have seen the effect of a busted pipe in the firebox, and what you suggest 
is the same as a busted feedwater pipe. The sudden influx of moisture(water) 
into the firebox, causes rapid expansion of that moisture. The firebox will 
be blasted with a sudden pressure surge, and may or may not rupture it's 
sides. The fire will go out, sure, but the damage will have already been 
done. Hopefully fire and parts don't get slung out of the building.
 
Better to have a low level shut off on the feed system
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