To stop a boiler leak, add a liquid or powder sealant to the water in the boiler to create a seal that grows with heat and is resistant to pressure. The sealant mixes with the water inside the boiler, sealing off any small holes or cracks.
It is important to secure adequate ventilation in the working area as sealant is dangerous to inhale. If ventilation is not possible, the user should put on a mask to keep the vapors out of his respiratory system. In either event, eye protection and gloves are a must for safety, as are long-sleeved shirts and pants to shield the skin.
Prepare the sealant following the manufacturer's instructions. If the boiler is new, fill it partially with water at normal temperature. Take off the valve or safety fitting, and pour the recommended amount of sealant. Close the safety valve, and allow the water to circulate to finish the job.
With an existing boiler, fill the boiler to 1/2 inch higher than the crack before pouring in the sealant. The opening nearest the leak is ideal for adding water, but the safety valve works if there are no closer openings. Apply the recommended amount of sealant, and close the opening. Build pressure to the highest amount allowed until the leak seals. If the leak is too large for this to work, it likely is time for a replacement.