Seals small leaks and weeps which may cause pressure loss and/or boiler breakdown. An internal leak sealer developed for use throughout a central heating system to seal small, inaccessible leaks and weeps which may cause pressure loss and boiler breakdown. Once dispersed within the system, the product finds the area of oxygen ingress and forms a polymer to seal the pinhole. Effective within 1-24 hours after application, the unique sealing efficiency of this water treatment chemical has been proven to prevent blockages or restrictions within the boiler or vulnerable system components such as pumps and safety devices. A leak can be quickly and efficiently sealed, without the need for a complete system drain down. Ideal for reducing pressure loss from sealed systems. Seals most weeps and small inaccessible leaks. Compatible with all metals and materials commonly used in central heating systems. One 500ml recyclable bottle treats up to 130 litres of system water (16 radiators) and the equivalent of 250m2 of underfloor systems. Fernox F4 Central Heating Leak Sealer 500ml. The F4 is suitable to be used throughout a central heating system to seal small leaks, and weeps which may cause pressure loss, and boiler breakdown. Once dispersed within the system it will be effective 1-24 hours after application. The unique sealing efficiency of this proven product has been specifically developed to prevent blockages, or restrictions within the boiler, or vulnerable system components such as pumps, and safety devices. Seal small leaks in central heating systems with Fernox F4 Leak Sealer. Supplied in a 500ml bottle, it targets inaccessible leaks in radiators, boilers, and pipework. Compatible with all system types and helps reduce pressure loss without damaging components.