I am planning to purchase a house built in 1949 with hot water heat. Im looking for an answer to your question, and if I find one, I will let you know. However, I also have a question for you. How do you know you have lime in your heating system? Can one just assume that any hot water system that is over 50 years old has lime deposits? I was told that if distilled water and antifreeze were used, there shouldn't be any problem, since antifreeze has additives that inhibit mineral deposition. How old are your pipes and radiators? Also, what metal are your pipes made of? I have a book written by a home inspector who says that galvanized steel pipes can rust from the inside. Rust particles then break off the inside and clog toilet refill valves and washer screens. Could your problem just be rust?