Hercules Boiler Liquid seals and repairs cracks and leaks in steam and hot water systems. Prevents steam boiler surging and will not clog or coat submerged hot water heating coils, controls, vents or valves. Plumbers have come to trust the Hercules offering to provide the highest quality of plumbing products. For use in low pressure 1 or 2 pipe steam, vapor or vacuum systems. Will not work in a cold boiler.