Hot water boilers don't need cleaning very often. A thorough cleaning at the time the system is installed is necessary, but after that it shouldn't be necessary to do so unless the water becomes obviously discolored or visible scale starts to form on the outside of pipes, valves or fittings.
Use 1 lb. sulphamic or hydrochloric acid-based cleaner and descaler per 50 gallons of water. You may be able to buy the proper chemicals from your boiler manufacturer, or from a boiler water treatment supply house.
Dissolve each pound of cleaner in one gallon of water. Pour the treated water into the air space of the hot water boiler's expansion tank.
Let the system fill, then vent the air from the expansion tank by opening the manual bleeder valve. Set the temperature at approximately 160 to 175 degrees. Let it run while filled with the cleaning solution for 2 to 4 hours.
Open the drain valve to let all cleaning solution drain out of the boiler and the system. Flush the boiler with clean water by opening up the manual water feed valve in order to remove all of the solution.
Fill the boiler again, vent it and set it to 150 degrees. Let it run for 30 minutes to 1 hour.
Drain the water again, and flush the system once more. Keep flushing and draining until the water comes out clear.
Refill the hot water boiler with water, and vent the excess air again. Your boiler should be set to resume normal operation.