Sentinel Water Treatment Salamander 22mm SESI Scale Inhibitor, limescale reducer, inline scale reducer, water conditioner, anti scale valve

Central heating inhibitors are chemicals added to the water in a wet heating system to protect the boiler, radiators and pipes against corrosion and scale. Unchecked corrosion forms black magnetite sludge, which leads to cold spots and can damage the boiler. This category covers liquid inhibitors in bottles and drums, concentrates and gels, rapid-dose cans, fluid for solar thermal systems, and in-line scale inhibitors.

Choosing and dosing

Dose to the volume of water in the system; many inhibitors treat about 100 litres of system water per litre, so larger systems need more. Check the product suits every metal present, including aluminium. On open vented systems the inhibitor goes in through the feed and expansion cistern; on sealed systems it is injected at the filling point, and gels and rapid-dose cans can go in through a radiator bleed valve with an adaptor.

When to add and test it

Add inhibitor with the final fill of fresh water, after the system has been cleaned and flushed. In England, Approved Document L expects this before a new boiler is fitted, with the system prepared and commissioned to BS 7593. That code of practice also calls for the water to be tested at commissioning and every year, and for the inhibitor to be re-dosed every five years unless a laboratory test shows it is still working. Recheck the level after any work that drains down or dilutes the system.

Solar thermal circuits use their own inhibited glycol fluid, supplied ready to use; do not dilute it or mix it with other fluids. Follow the manufacturer's instructions and safety data. Work on the boiler's gas parts must only be carried out by a Gas Safe registered engineer.

Before dosing, see central heating cleaner, or protect the boiler further with central heating filters.