How to add central heating inhibitor to your system

Central heating inhibitor is a chemical treatment that prevents internal corrosion and scale formation in the boiler heat exchanger, pump, pipework, and radiators. Without it, magnetite sludge (black iron oxide) builds up rapidly — particularly in steel radiators. UK boiler manufacturers require inhibitor to be present to maintain warranty.

On a sealed (pressurised) system, the easiest way to add inhibitor is via a radiator bleed valve. First, identify the smallest or highest radiator (you will add inhibitor here). Turn off the radiator's TRV (thermostatic valve) and lockshield valve, then bleed out approximately 500ml of system water into a container using the bleed key.

Pour the inhibitor into a dedicated inhibitor applicator bottle (a hand-pump tool that forces liquid in through the bleed valve under pressure) or use the type of inhibitor that comes in a pressurised can for direct injection. Follow the manufacturer's dosing instructions — typically one 500ml bottle treats up to 100 litres of system water (about 10 radiators).

Alternatively, on an open-vented system (with a small header tank in the loft), simply pour the inhibitor directly into the header tank. It will circulate throughout the system when the heating runs.

After adding inhibitor, restore the radiator valves, repressurise the system if the pressure dropped (via the filling loop), and run the heating for a full cycle to distribute the inhibitor throughout.

Test the inhibitor concentration annually with a test strip (Fernox and Sentinel both sell these, £5–£8 per pack). Top up as needed, typically every 2–3 years or after any system work that requires draining. Replace fully every 5 years or after a power flush.