> There are many types of radiator heating among which is high pressure
 > steam as you noted. Generally high pressure steam is used for larger
 > dwellings such as multi-apartment housing.
I was responsible for maintaining a couple of buildings in my previous job. 
One was a factory of 170,000 square feet, the other a 10-story building of 
300,000 square feet. Both had low-pressure steam boilers. The factory boiler 
operated at 10 PSI, the tall building at 5 PSI or so.
Dunno the details of the regulations in all locales, but I think that 
anything under 15 PSI is considered "Low Pressure" and the operator 
liscensing requirements are MUCH more lenient than for high-pressure boilers. 
It takes quite a lot of study to get a liscense for the latter; my wife had 
to get one when she worked for Sun Oil Company and she spent a fair amount of 
time studying. The guy who held the liscense for the low pressure boilers 
where I worked wasn't smart enough to EVER get a high-pressure liscense.
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