ComStar s Boiler Seal and Liquid Plug are both non-freezing, odorless and quick-acting sealants for use in steam boilers and hot water systems. They can be used in both new installations and existing systems. When Boiler Seal or Liquid Plug is added to the boiler water, it rapidly seeks out the leak, fills the crack and then hardens to seal the opening and stop the leak. Boiler Seal and Liquid Plug will not affect existing seals in the system and can be left in the system. For small openings or when location of leak is unknown, use Liquid Plug.