My oil company technician recently came over to do an annual cleaning of our steam boiler and told me it is not necessary to drain the boiler of dirty water (since it will not remove any of the water that is caked on the internal parts, anyway). He actually said it is more harmful to bring fresh water into the plumbing because it introduces more minerals that break down the cast iron pipes.
This conflicts information I've heard from other technicians. One told me to drain a gallon of water once per week, another told me to completely drain the system once per month.
What is the right answer here?