Yea hootyowl, I have to agree with you. CLR in a HEALTHY system can work wonders. But some of the really old buildings that we owned outside of Philly sported some of the most ancient piping you could imagine. And yea, after a treatment, we had some pipes that looked like garden sprinklers.
Lesson learned.
But, we had our own pipe cutting and threading tools, so we just cut off the bad parts and spliced in the new.
Sometimes though, it was more economical and practical to just overhaul the whole system, run new pipes, get the radiators to a pro and have them cooked out. Lots of work, but when you get an old cast iron system up and running like it was new, it will last another 100 years and give you some good low cost heat.
We tried switching to the copper base boards on one place. Mistake. The copper cooled way too fast (the cast radiators [and the black pipe] radiated heat long after the circulator pump shut down) and the fuel oil costs were noticeably higher.
Nothing at all wrong with the old systems if you know how to maintain them. And the bottom line, it really isn’t that hard or expensive.