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Not sure where there has been a discussion — but the overall topic of operating a steam system in a vacuum does come up from time to time.

And it is quite true — the boiling point of water changes, as you would expect, with absolute pressure, so depending on how deep the vacuum is you can run significantly lower temperatures.

There are a couple of problems which kind of complicate things. The first is achieving a vacuum in the first place. In the bad old days we kind of did that in a backwards way: the boiler was brought up to steam and the air allowed to vent out of the system — but these weren't garden variety of vents. Rather, they sealed to hold a vacuum. So as the fire was allowed to die down (we're looking at coal here) the system naturally went into a vacuum and allowed prolonged operation at rather low temperatures. Vacuum type vents are still available — but are pricey. Some folks have experimented with adding check valves to regular vents to accomplish the same purpose. These were generally two pipe systems running on very small differential pressures (what are called vapour systems), but the same principle could be applied to one pipe systems — with the need for the specialized vents or check valves on every radiator.

One can also have systems which have mechanically induced vacuum. This adds a good bit of complexity to the system, particularly if the system isn't perfectly vacuum tight.

OK, I actually delivered 286 Gallons to a customer that only had a 275 Gallon tank. Their basement floor looked eerily like the one in the news story. This customer's tank was reported to have a partially clogged vent pipe and therefore the tank needed to be filled slowly. Each time I delivered to this tank, I turned in a request to have a service technician fix the clogged vent pipe. Each time the service tech looked at the problem, they would connect the soot vac exhaust to the fill pipe and the vent alarm whistle worked just fine. So the vent pipe was never taken apart and inspected for a blockage.

One day while delivering to this customer, I forgot to set the delivery truck pump speed to SLOW FILL. After I was there for more time than usual, I looked at the meter on the truck and read 287 Gallons. I closed the fill nozzle immediately. The whistle was still whistling after the oil stopped. The back pressure in the tank was enough to cause the whistle to keep sounding for a good 35 seconds after I stopped filling the tank. I knocked on the customers door and asked if I could look at something in the basement.  There was fuel oil creeping under the closed doorway of a finished basement. 

O well!

I made a radio call to the office that thor tank split and we needed to get right over there to clean up the oil leaking from the tank.  That customer switched to natural gas rather than get a new oil tank. This happened in the 1970s when oil spills were not an EPA disaster.  But we did our best to clean up the mess and made the necessary insurance claim.    

Cast iron radiators do not "turn off".

Nor do the entire radiators reach 212 except under extreme conditions, generally a few sections do while the boiler is running and then the hotter sections dissipate some into the cooler sections. This allows the massive radiators to keep heating the space while the boiler is off. The system doesn't behave like it's turning on and off, similar to a concrete radiant floor, just not as massive obviously.

My next question is, you say 200F is ok but 12 degrees hotter isn't? Can you explain please? That's a 6% increase…….

Some on this thread addressed shoddy modern building. My biggest gripe as an HVAC guy is insulation installed by people who have no clue or maybe it's no care. Case in point; I was called back to a floor heat job we did with the complaint of a cold powder room. Odd, I thought. because the room was totally interior with the only heat loss being the ceiling. I walked in the room, laid my hand on one wall, then the other. One cold, one room temp. Being an addition, the cold wall was studs on the old house brick with no top plate and wide open to the attic, turning the whole wall into an uninsulated "exterior" wall. I see too many such cases where it takes almost nothing to insulate right (vs what was done) but there is no thinking or no care.