Fernox F4 Leak Sealer 500 ml (formerly LSI) is a specialist central heating system additive designed to stop and prevent internal water leaks in sealed heating systems. It’s formulated to travel with the circulating water and seal small leaks or weeps in radiators, pipework, and boiler heat exchangers without blocking normal flow.

Function & Purpose

F4 Leak Sealer works by depositing sealing polymers at sites of minor leaks and drips within a heating system. When water escapes through small gaps in metal or joints, the pressure change causes the sealing agents in F4 to activate and collect at the leak location, forming a flexible, durable seal. This helps reduce or eliminate slow weeps or drips that can lead to loss of system water, pump noise, corrosion, and pressure issues.

Compatibility

This product is suitable for use in sealed central heating systems including combi, system, and conventional boilers. It is compatible with metal pipework (such as copper and steel) and radiators when used as directed. Fernox F4 can be used alongside corrosion inhibitors and conditioners, but always follow instructions regarding sequencing (typically add F4 after initial cleaning and inhibitor dosing).

How It Works

The 500 ml bottle is added directly to the system via a filling point such as a filling loop or magnetic filter connection. Once introduced, the system is run at normal operating temperature so the product circulates and the sealing action can take effect. Over time, the leak sealer collects at leak sites and hardens to form a flexible, lasting seal without impeding normal water flow.

Key Benefits

F4 Leak Sealer provides a non-invasive way to address small leaks without the need for immediate drain-downs or component replacement. It helps reduce water loss, maintain system pressure, minimise the need for repeated top-ups, and avoid corrosion that can accompany persistent moisture and oxygen ingress. It can save time, prevent damage, and extend system life by keeping water where it belongs.

Ideal Applications

This leak sealer is ideal for central heating systems that suffer from minor internal leak issues, such as hairline cracks in radiators, slight weeping at joints, or small pinhole leaks in boiler heat exchangers or pipework. It’s often used by heating engineers and homeowners as a preventative or corrective maintenance product, particularly in hard-to-access areas where mechanical repair is difficult.

Usage Note

Always follow the manufacturer’s instructions for dosing and circulation time. F4 Leak Sealer is designed for minor internal leaks and is not a substitute for mechanical repair of major faults. After treatment, monitor system pressure and performance, and consider a professional inspection if leaks persist.