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
Power Cleaner F8 is a concentrated formulation supplied in 500ml bottles, to reduce packaging waste. Just one bottle treats larger systems of 130 litres, 16 radiators or 250 sqm of underfloor systems and one dose is sufficient for cleaning the central heating systems of up to 97% of all UK homes (or larger heating systems).
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
