Best way to clean calcium build up out of a radiator??

Not to run tap water to begin with. Run distilled water like what's supposed to be used. You'll never have calcium buildup if you never introduce calcium in the system.

That said, I have had GREAT luck draining the system, refilling it with water (you CAN use tap water for this) and adding a WHOLE bottle of CLR and driving it a few days. CLR:

http://www.uline.com/BL_7097/CLR-Ca...utm_term=clr&utm_campaign=Janitorial Supplies

You don't need a gallon. Just get whatever Wally World has. That will be enough.