Central heating inhibitor — what it is and how to add it

Central heating inhibitor is a chemical added to your system water that prevents internal corrosion and sludge formation. Without it, steel radiators and cast iron components corrode, producing black iron oxide sludge (magnetite) that blocks the system and reduces efficiency.

You should add inhibitor when: a new system is commissioned, after a powerflush, after adding a new radiator, or if the system has been drained and refilled. Inhibitor concentration diminishes over time and should be checked annually at your boiler service.

The most widely used products are Fernox F1 Protector and Sentinel X100. For a typical 10-radiator system, one 500ml bottle is sufficient. Follow the bottle instructions for larger systems.

The easiest way to add inhibitor is via a radiator: identify the highest radiator on the system (usually upstairs). Place a towel and bucket beneath the bleed valve, open the bleed valve and insert the inhibitor bottle nozzle, then squeeze the contents in. Close the bleed valve, top up system pressure to 1–1.5 bar, and run the heating to distribute the inhibitor.

Alternatively, you can pour inhibitor into the feed and expansion tank (old open-vented systems) or use a dosing pot installed on the return pipe.

If you have a magnetic filter fitted (Fernox TF1, Adey MagnaClean), clean it annually: isolate the valves either side, open the cap, wipe the magnet clean of sludge, rinse, and refit. This single action can extend your boiler's life significantly.