• 30-Second Application: Applies in just 30 seconds with no system drain-down required, helping you quickly seal leaks and restore pressure with minimal disruption.
• Safe for All Heating Components: This radiator leak sealant is non-toxic and safe for boilers, pumps, air vents, and heat exchangers. Will not cause blockages, making it ideal for sealed and combi systems.
• High System Coverage: One 400ml can treats up to 130 litres (around 16 radiators) or 250m2 of underfloor heating providing wide coverage for minor leaks in sealed systems.
• Radiator Stop Leak Performance: Seeks out inaccessible micro-leaks caused by air ingress and seals them without blocking heat exchangers or pumps. Ideal as a radiator stop leak for central heating systems.
• Low-Impact Leak Sealant Spray: Powered by compressed air instead of harmful propellants, this leak sealant spray is non-flammable and comes in a fully recyclable can.
• For Trade or DIY Maintenance: Trusted by heating engineers and DIY users alike, this leak sealer makes in-system leak repair quick and hassle-free. Ideal for maintaining pressure in sealed systems.

LavaTap - Fernox F4 Express Leak Sealer is a fast-acting internal leak sealant for minor leaks in central heating systems. Specially designed for sealed and combi systems, this 400 ml aerosol solution works without the need to drain the system, saving time, water, and labour. It quickly locates and seals hidden micro-leaks in radiators, pipes, joints, and heat exchangers by forming a durable polymer seal wherever air ingress is detected. This leak sealer is suitable for use with all standard central heating system materials and is safe for boilers, pumps, and valves. The air-powered aerosol can be applied directly into the system via a radiator, filling loop, or TF1 filter using the included universal adapters. With a treatment capacity of up to 130 L of water (or 16 radiators), it is an efficient choice for household and light commercial systems. Unlike traditional radiator sealants, Fernox F4 Express is environmentally responsible-it uses compressed air instead of chemical propellants and comes in a recyclable container. Ideal for heating engineers and DIY users alike, it helps prevent damage caused by water loss and pressure drops, offering a convenient stop leak solution until permanent repairs can be made. For best results, ensure system cleanliness and avoid using in heavily sludged systems. Not suitable for repairing major leaks or burst pipes.