To clean a cast iron boiler, a heating professional can:
Use one pound of trisodium phosphate for every 50 gallons of water content.
Fill, vent and circulate the system with above mixture, allowing it to reach design or operating temperature if possible.
After circulating a few hours, drain the system completely and refill with fresh water. Usually, enough of the cleaner will adhere to the piping to give an alkaline solution satisfactory for operation: a pH reading between 7 and 8.5. Sodium carbonate (up to one pound for every 30 gallons of water content) or sodium hydroxide (lye) (up to one pound for every 50 gallons) may be used.
Check pH level after cleaning. It should be between 7 and 8.5. A small amount of cleaner maybe added to adjust the pH up to the proper range.
DO NOT use petroleum-based products in the boiler.