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 Fernox Express range uses Barrier Pressure Pack technology and compressed air propellent to expel the product so the entire range is non-flammable. The propellent (compressed air) is not mixed with the active product so there is no risk of air ingress into the system which could accelerate future risk of corrosion.
A single dose of Leak Sealer F4 Express treats a larger sized property with a heating system of 34 gallons, up to 16 radiators or 250 sqm of underfloor heating and can be dispensed directly into a radiator air vent, system filling loop, or via a TF1 Filter using the appropriate adapter supplied with the product. Fernox Express takes less then 30 seconds to dose the system without the need to drain down the system, saving time on-site. If dosing via a TF1 Filter, the filter should be emptied of any collected contaminants prior to treating with the product. In single feed indirect cylinder, e.g. ‘Primatic’ or similar, potable water chemicals must be used.
Form: Viscous liquid
pH: 5.0 – 7.0
SG: 1.0 @ 20°C
Single Unit Dimensions
Single Unit Barcode
Outer Carton Dimensions
Outer Carton Barcode (ITF)
95 × 66 × 218 mm
241 x 178 x 339 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.
Make sure the system is turned off and allow radiators to cool down (1). Select a radiator and shut both the wheel-head and lock shield valves (2). If a thermostatic radiator valve (TRV) is fitted, close it by turning it to the minimum setting.
Having removed the air vent pin using a cloth to absorb any escaping water (3). Screw the Fernox Express injection nozzle into the air vent (after cutting to the right diameter to fit tightly (4)). Fit the tube to the adapter and screw onto the canister. Attach the other end of the tube onto the nozzle in the radiator (5).
Re-open the wheel-head valve (6) (or TRV) and inject all the content of the Fernox Express by squeezing the trigger for 30 seconds (7). After the product has been dispensed close the wheel-head valve (8) (or TRV), remove the injector nozzle and tighten the air vent pin (9). Open both radiator valves, turn on the central heating system (10) and let it run for 15 minutes to allow the Express to disperse around the system.
Fernox Express can also be for dosed via the system filling loop using the appropriate special adapter supplied with the Express product as well as via the bleed valve on a fitted Fernox TF1 Filter. The whole process takes less than two minutes.