Leak Sealer F4 is a 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 market-leading 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.
The recommended ‘in use’ concentration of the product is 0.385%. One single dose of this product treats a larger sized property with a heating system of 130 litres, up to 16 radiators or 250 sqm of underfloor heating. For open vented systems add via the feed and expansion tank. For sealed systems add via a suitable dosing point (e.g. a towel rail or an in-line system filter). Alternatively, use a suitable dosing vessel to inject the product into the system. In single feed indirect cylinders, e.g. ‘Primatic’ or similar, potable water chemicals must be used.
Fernox Leak Sealer F4 is supplied in 500 ml bottles.
Fernox Leak Sealer F4 is classified as non-hazardous, but as with all chemicals, keep out of reach of children. In case of contact with eyes or skin, rinse immediately with plenty of water.
Fernox Leak Sealer F4 is a polymer emulsion.
Colour: Off white
Form: Viscous liquid
pH (conc.): 5.0 – 7.0
pH (in-use): 7.0 – 8.5
SG: 1.0 at 20°C
Single Unit Dimensions
Single Unit Barcode
Outer Carton Dimensions
Outer Carton Barcode (ITF)
52 × 88 × 182 mm
195 x 265 x 180 mm
Yes, all Fernox internal leak sealers, such as Fernox Leak Sealer F4, are safe for use with combination and other low water content boilers.
They will not block valves or vents. Leak Sealer F4 Express is recommended for all pressurised systems as this can be added in 30 seconds via a radiator, filling loop or Fernox TF1 Filter, without the need to drain the system.