> When I moved in this house 28 years ago, it had a coal fired hot
 > water boiler converted to natural gas using a "columbia fire
 > ring". The boiler was in the basement with several large cast iron
 > radiators downstairs and upstairs and the expansion tank in
 > the attic. It was termastatically controlled but had a manual
 > switch in case of power loss. Years later I scrapped it all out
 > thinking the radiators were so old that they were probably
 > plugged, they were very clean inside. I could have just put
A steam or hot water system should stay very clean inside as long as there 
weren't any leaks that caused constant water replacement. The oxygen is 
boiled out of the water, and the minerals deposit themselves on a one-time 
basis. So no rust or scale.
Bring in new water with new minerals and oxygen and you can kill the system 
quickly, especially without proper water treatment such as chemicals and 
pre-heat in a vented tank.
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