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.
Normally a treatment will last the life of the system but an annual check to determine whether further dosing is necessary is recommended. If the system is drained, it will need to be re-treated with a further full dose of Protector. You can check if the system is properly dosed using a Fernox Protector Test Kit.
Fernox Cleaner F3 should ideally circulate through the system for at least one hour, or up to a week. Afterward, it should be drained, and the system flushed and cleaned before adding Fernox Protector F1 or Protector + Filter Fluid, together with a Fernox TF1 Filter, to protect the system from further corrosion and scale formation.
You should only drain central heating system water into the foul water drain or a septic tank. Never drain system water into road drains or rain water gullies as these go directly into rivers. Never pour water from a central heating system onto the garden or lawn.
